Laser-based material processing methods and systems

ABSTRACT

Various embodiments may be used for laser-based modification of target material of a workpiece while advantageously achieving improvements in processing throughput and/or quality. Embodiments of a method of processing may include focusing and directing laser pulses to a region of the workpiece at a pulse repetition rate sufficiently high so that material is efficiently removed from the region and a quantity of unwanted material within the region, proximate to the region, or both is reduced relative to a quantity obtainable at a lower repetition rate. In at least one embodiment, an ultrashort pulse laser system may include at least one of a fiber amplifier or fiber laser. Various embodiments are suitable for at least one of dicing, cutting, scribing, and forming features on or within a semiconductor substrate. Workpiece materials may also include metals, inorganic or organic dielectrics, or any material to be micromachined with femtosecond and/or picosecond pulses, and in some embodiments with pulse widths up to a few nanoseconds.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §120 and 35 U.S.C.§365(c) as a continuation of International Application No.PCT/US2009/037443 designating the United States, with an internationalfiling date of Mar. 17, 2009, entitled “LASER-BASED MATERIAL PROCESSINGMETHODS AND SYSTEMS,” which claims the benefit under 35 U.S.C. §119(e)to U.S. Provisional Patent Application No. 61/038,725, filed Mar. 21,2008, entitled “LASER-BASED MATERIAL PROCESSING METHODS AND SYSTEMS,” toU.S. Provisional Patent Application No. 61/110,913, filed Nov. 3, 2008,entitled “LASER-BASED MATERIAL PROCESSING METHODS AND SYSTEMS,” and toU.S. Provisional Patent Application No. 61/152,625, filed Feb. 13, 2009,entitled “LASER-BASED MATERIAL PROCESSING METHODS AND SYSTEMS,” theentire disclosures of each of the aforementioned internationalapplication and provisional applications are hereby incorporated byreference herein in their entirety.

This application is related to co-pending international patentapplication number PCT/US08/51713, filed Jan. 22, 2008, entitled“ULTRASHORT LASER MICRO-TEXTURE PRINTING,” published as internationalpublication no. WO 2008/091898, which claims the benefit of U.S.Provisional Patent Application No. 60/886,285, filed Jan. 23, 2007,entitled “ULTRASHORT LASER MICRO-TEXTURE PRINTING.” This application isalso related to U.S. patent application Ser. No. 10/813,269, filed Mar.31, 2004, entitled “FEMTOSECOND LASER PROCESSING SYSTEM WITH PROCESSPARAMETERS, CONTROLS AND FEEDBACK,” now U.S. Pat. No. 7,486,705. Each ofthe above-identified patent applications, publication, and patent isowned by the assignee of the present application. The disclosures ofeach of the above-identified applications, publication, and patent arehereby incorporated by reference herein in their entirety.

BACKGROUND

1. Field

This disclosure relates generally to pulsed lasers and machiningmaterials using high repetition rate pulsed lasers.

2. Description of the Related Art

Several material processing applications including, for example, thinsilicon wafer dicing, printed circuit board (PCB) drilling, solar cellmanufacturing, and flat panel display manufacturing, involve similarmaterial processing techniques and problems. Early solutions includedmechanical and lithographic processing techniques. However, thereduction in device size, increased device complexity, and theenvironmental cost of chemical processing transitioned the industrytoward laser processing methods. High power diode-pumped solid statelasers having typical wavelengths of 1 μm, or frequency convertedversions having green or UV wavelengths, are now utilized. One methodutilized in some applications includes progressively cutting through thematerial with repetitive passes at relatively high scanning speeds. Insuch applications, there are three main problems: (a) cleaning cuttingthrough the desired material without causing damage to the material(e.g., residual stress, delamination, thermally induced materialmodification, etc.), (b) achieving a sufficiently high volume materialremoval rate to be commercially viable, and (c) reduction/elimination ofrecast material.

Various options have been suggested for efficient and high-qualitylaser-based machining of materials, including operation at highrepetition rates with less debris and melt. However, the problem oflimiting accumulation of re-deposited material near a processing sitehas not been sufficiently addressed, and is generally a difficultproblem to overcome. As high material removal rates are required forrapid processing, the relatively large amount of ablated materialejected from a processing site may generally include one or more ofliquid melt, relatively large quantities of solid material, and vapor.Fine distributions of particles, down to the nanometer scale (e.g., 10nm), may also be redeposited.

In various applications, the problem of limiting accumulation has beenaddressed with process modifications. For example, in some currentsemiconductor-industry techniques, a substrate may be coated with asacrificial layer of material that is removed with the redepositedmaterial after laser processing. This process step may be used alone orin combination with post-processing of the substrate with variouschemical solvents to remove the recast. However, such techniques reducethroughput and increase costs by adding additional processing steps andadditional consumable materials. As such, a preferred solution wouldeliminate the need for such debris removal.

Process debris may include slag, melted regions, heat-affected zones(HAZ), and so forth. In some cases, the debris cannot be effectivelyremoved using conventional non-chemical cleaning techniques such as, forexample, cleaning in an ultrasonic bath.

Moreover, low-k material and composite layers utilized in integratedcircuits and semiconductor devices introduce challenges for certainimplementations of laser-based material processing. Low-k material caninclude material that has a dielectric constant that is less than thedielectric constant of silicon dioxide. For example, low-k material caninclude dielectric materials such as doped silicon dioxide, polymericdielectrics, etc.

SUMMARY

Because of the foregoing challenges and limitations, the inventors haverecognized a need exists not only to efficiently machine materials butalso to limit accumulation of redeposited material. Solutions whichwould eliminate expensive processing steps are highly desirable.Therefore, various embodiments of the systems and methods disclosedherein may be used for laser-based modification of target material of aworkpiece while simultaneously achieving improvements in processingthroughput and/or quality.

In one general aspect, a method of laser processing a workpiece isprovided. The method may comprise focusing and directing laser pulses toa region of the workpiece at a pulse repetition rate sufficiently highso that material is efficiently removed from the region and a quantityof unwanted material within or proximate to the region is reducedrelative to a quantity obtainable at a lower repetition rate. Forexample, the pulse repetition rate may be in a range from about 100 kHzto about 5 MHz in some embodiments of the method.

In another general aspect, a method of laser processing a workpiece maycomprise focusing and directing laser pulses to a region of theworkpiece at a pulse repetition rate sufficiently high so that heataccumulation within one or more materials is controlled in such a waythat provides for rapid material removal, while limiting accumulation ofredeposited material about the processed area. The method may allowcontrol of a heat-affected zone (HAZ).

In another general aspect, a method of laser processing a workpieceincludes irradiating at least one material of the workpiece with laserpulses having a pulse width. The laser pulses may be focused onto spotsin the at least one material. The focused spots may be relativelyscanned with respect to the material at a scanning rate. In someimplementations, the workpiece comprises a patterned region and a baresemiconductor wafer region. The patterned region can comprise at leastone of a dielectric material and a metal material. In some embodiments,the scanning rate used for removal of at least a portion of thepatterned region is substantially less than the scanning rate used forremoval of at least a portion of the bare wafer region.

In some embodiments, an overlap between adjacent focused spots issubstantially greater for irradiation of the patterned region than forirradiation of the bare wafer region. For example, the overlap forirradiation of the patterned region may be greater than about 95% insome cases.

In some embodiments, at least a portion of material within the patternedregion is modified using a pulse width in a range of about 100 ps toabout 500 ps. In some embodiments, at least a portion of material withinthe semiconductor wafer region is modified using a pulse width in arange of about 100 fs to about 10 ps.

At least one implementation includes an ultrashort pulse laser systemsuitable for carrying out embodiments of the above methods of laserprocessing. At least one embodiment includes an ultrashort pulse lasersystem that comprises at least one of a fiber amplifier or a fiberlaser. At least one embodiment includes an ultrashort pulse laser systemconfigured as an “all-fiber” design.

In various embodiments, a pulsed laser system provides a pulse width ofat least one pulse that is less than about 10 ps In some embodiments, apulse width of at least one pulse is less than about a few nanoseconds,for example a sub-nanosecond pulse.

Embodiments of a method of scribing, dicing, cutting, or processing toremove material from a region of a multi-material workpiece areprovided. In some embodiments, the method comprises directing laserpulses toward at least one material of a multi-material workpiece. Thelaser pulses can have a pulse width in a range from tens of femtosecondsto about 500 picoseconds and a pulse repetition rate of a few hundredkHz to about 10 MHz. The workpiece can comprise both a pattern and asemiconductor wafer, and the pattern can comprise at least one of adielectric material and a metal material. The method can also includefocusing the laser pulses into lasers spots having spot sizes in a rangefrom a few microns to about 50 μm (1/e²) and positioning the laser spotsrelative to the at least one material at a scan speed such that anoverlap between adjacent focused spots for removal of material from atleast a portion of the pattern is substantially greater than an overlapbetween adjacent focused spots for removal of material from at least aportion of the semiconductor wafer. In certain advantageousimplementations, the method controls heat accumulation within one ormore materials of the workpiece, while limiting accumulation ofredeposited material about the region.

Embodiments of a method of processing a workpiece that comprises apattern and a semiconductor wafer are provided. The pattern can compriseat least one of a dielectric material and a metal material. In someembodiments, the method includes modifying at least a portion of thepattern with a laser pulse comprising a pulse width in the range fromabout 100 ps to about 500 ps and modifying at least a portion of thesemiconductor wafer with a laser pulse comprising a pulse width in arange from about 100 fs to about 10 ps.

Embodiments of a method of laser processing a multi-material workpiecehaving a semiconductor material are provided. In some embodiments, themethod comprises focusing and directing laser pulses to a region of theworkpiece at a pulse repetition rate in a range from about 100 kHz toabout 10 MHz and at a repetition rate sufficiently high so that materialis efficiently removed from the region and a quantity of unwantedmaterial within or proximate to the region is limited relative to aquantity obtainable at a lower repetition rate below about 100 kHz.

In other embodiments, methods of laser processing a multi-materialworkpiece having a semiconductor material are provided. In some suchembodiments, the method comprises repeatedly irradiating at least onetarget material of the workpiece with focused laser pulses at a scanrate and a pulse repetition rate. The repetition rate may be in a rangeof at least about a few hundred kHz to about 10 MHz, and the scan ratemay be in a range of about 0.2 m/s to about 20 m/s. In variousembodiments of the method, at least some of the focused laser pulseshave a non-zero spatial overlap factor with at least one other pulse, apulse width less than about 1 ns, a pulse energy in a range of about 100nJ to about 25 μJ, a focused (1/e²) spot size in a range of about 5 μmto about 50 μm, and a fluence in a range of about 0.25 J/cm² to about 30J/cm² at the target material.

Embodiments of method of processing a multi-material workpiece aredisclosed. The workpiece can comprise a semiconductor material and apattern, and the pattern can comprise at least one of a dielectricmaterial and metal material. In some embodiments, the method comprisesirradiating the workpiece with a series of laser pulses, with at leasttwo pulses of the series having different characteristics that areapplied to different materials of the workpiece. The method alsocomprises controlling heat-affected zone (HAZ) such that at least oneHAZ generated during removal of at least one of the dielectric materialand the metal material is increased depthwise relative to at least oneHAZ generated during removal of a portion of the semiconductor material.

Embodiments of a method of processing a workpiece comprising both apattern and a semiconductor wafer region are disclosed. The pattern cancomprise a dielectric material and a metal material. In someembodiments, the method comprises modifying at least a portion of thepattern with focused laser pulses, with at least one focused pulsecomprising a pulse width in a range of about 100 fs to about 500 ps. Themethod also includes accumulating sufficient heat in the portion of thepattern to avoid delamination of the dielectric material from the metalmaterial.

Embodiments of a laser-based system for scribing, dicing, cutting, orprocessing a multi-material workpiece having a semiconductor materialare provided. In some embodiments, the laser-based system comprises asource of optical pulses and an optical amplification system configuredto amplify a pulse from the source to a pulse energy of at least about 1μJ and to generate output optical pulses having at least one pulse widthin a range from about 500 fs to a few hundred picoseconds. The systemmay also include a modulation system, comprising at least one opticalmodulator, configured to adjust a repetition rate of the output opticalpulses to be within a range from about 100 kHz to about 10 MHz, and abeam delivery system configured to focus and deliver pulsed laser beamsto the workpiece, such that a pulsed beam is focused into a spot size(1/e²) in a range from about 15 μm to about 50 μm. The system may alsoinclude a positioning system configured to scan the beams relative tothe one or more materials of the workpiece at a scan rate in a rangefrom about 0.1 msec to about 20 msec, and a controller configured to becoupled to at least the positioning system. The controller can beconfigured to control a spatial overlap between adjacent focused beamsduring processing of the workpiece at the repetition rate.

Embodiments of a laser-based system for scribing, dicing, cutting, orprocessing of a multi-material workpiece having a semiconductor materialare described herein. Embodiments of the system comprise a source ofoptical pulses and an optical amplification system configured to amplifya pulse from the source and to generate output pulses having at leastone pulse width in a range from tens of femtoseconds to about 500picoseconds. The system can also include a modulation system, includingat least one optical modulator, configured to provide a repetition rateof the output optical pulses to be in a range from at least about 1 MHzto less than about 100 MHz. The system also can include a beam deliverysystem configured to focus and deliver pulsed laser beams to theworkpiece, such that a pulsed beam is focused into a spot size (1/e²) ofat least about 5 microns, and a positioning system configured to scanthe beams at a scan rate that produces a spot overlap on or within theone or more materials of the workpiece. The spot overlap in variousimplementations may be at least about 95% at the repetition rate and thespot size.

Embodiments of a system for dicing, cutting, scribing, or formingfeatures on or within a workpiece having a semiconductor material areprovided. In some embodiments, the system comprises a pulsed lasersystem configured to repeatedly irradiate at least a portion of thematerial with focused laser pulses at a scan rate and a pulse repetitionrate. The repetition rate can be in a range of about 100 kHz to about 5MHz and sufficiently high to efficiently remove a substantial depthwiseportion of material from a target location and to limit accumulation ofunwanted material about the target location. The system can also includea beam delivery system configured to focus and deliver the laser pulses,and a positioning system configured to position the laser pulsesrelative to the semiconductor substrate at the scan rate. Thepositioning system can comprise at least one of an optical scanner and asubstrate positioner. In some embodiments, a controller is configured tobe coupled to the pulsed laser system, the beam delivery system, and thepositioning system. The controller can be configured to control aspatial overlap between adjacent focused laser pulses during processingof the workpiece at the repetition rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C schematically illustrate top and cross-sectional viewsrepresenting an embodiment of a multiple pass laser-based method formaterial removal.

FIGS. 1D-1E schematically illustrate cross-sectional views representinga relation between machining depth and formation of unwantedre-deposited material after N passes. FIG. 1E is a schematicrepresentation of a result obtainable with at least one embodiment of apulsed laser system.

FIG. 1F schematically illustrates an embodiment of a laser systemsuitable for processing a workpiece with laser pulses.

FIGS. 1G-1 to 1G-3 schematically illustrate examples of portions ofpatterned wafers. FIG. 1G-1 illustrates a wafer having several die, andFIG. 1G-2 illustrates an expanded view of a portion of the wafer of FIG.1G-1, and FIG. 1G-3 illustrates a cross sectional side-view of a portionof the wafer.

FIGS. 2A-2B schematically illustrate embodiments of a system forprocessing a workpiece with laser pulse trains.

FIG. 3 schematically illustrates another embodiment of a system forprocessing a workpiece with laser pulse trains

FIG. 4A schematically illustrates yet another embodiment of a system forprocessing a workpiece with laser pulse trains.

FIG. 4B schematically illustrates an embodiment of a large mode areafiber comprising a core doped with rare earth ions that can be used in afiber amplifier or in a laser pumped by a multimode pump source

FIG. 5 schematically illustrates a further embodiment of a system forprocessing a workpiece with laser pulse trains, the system havingfeedback and controls based on process and/or target information.

FIGS. 6A and 6B show a schematic representation and a photograph,respectively, illustrating an experimental system corresponding to anembodiment for processing a workpiece with laser pulse trains.

FIG. 7 schematically illustrates one example technique for quantifyingprocessing quality so as to obtain an approximation of an ablated volumeand a redeposited volume proximate to a processing location.

FIGS. 7A-7F show example scanning electron microscope (SEM) crosssections obtained from silicon samples, wherein the experimental resultswere obtained by varying laser processing parameters of the examplesystem of FIGS. 6A and 6B.

FIG. 8 is a plot showing examples of ablated cross-sectional area versusre-deposited cross sectional area as a function of scan speed andrepetition rate.

FIG. 9 is a plot of further illustrating examples of cross-sectionalarea versus scan speed, normalized for average power and spatial overlapof spots.

FIGS. 10A-1 and 10A-2 show example SEM cross-sections, wherein aquantity of re-deposited material is sufficiently low such thatconventional ultrasonic cleaning is effective for further debrisremoval, the result being applicable to, for example, thin-wafer dicingand similar applications.

FIG. 10B is a plot of a ratio of ablated depth to recast heightcorresponding to the data shown in FIGS. 10A-1 and 10A-2.

FIGS. 11A-11C show example SEM cross-sections comparing results ofsingle and double pulse processing.

FIGS. 11D-11E are plots showing the ratio of ablated depth to recastheight, corresponding to the SEM images of FIGS. 11A-11C.

FIGS. 12A-12B are SEM images showing a portion of a wafer cut (diced)using an embodiment of a pulsed laser system, and a result obtainedafter conventional ultrasonic cleaning.

FIGS. 13A-1-13A-3 are SEM images showing results obtained with variousrepetition rates and scan speeds using about 200 ps pulse widths.

FIGS. 13A-4-13A-5 are plots showing weighted ablated cross-sectionalarea (in square microns) and a ratio of ablated depth to recast height,respectively, corresponding to the data shown in FIGS. 13A-1-13A-3.

FIGS. 14 and 14A-1 and 14A-2 schematically illustrate various examplesof configurations used to test die strength of semiconductor devices,and FIGS. 14B and 14C are plots illustrating examples of die strengthmeasurements obtained after processing samples with ultrashort pulsesfrom the example experimental system illustrated in FIGS. 6A and 6B.

FIGS. 15A-15D show examples of SEM images, and cross sections of samplesscribed and/or cut with ultrashort pulses generated with the exampleexperimental system of FIGS. 6A and 6B.

FIGS. 16A-16D show examples of SEM images illustrating femtosecond andpicosecond scribing results.

FIG. 17 illustrates experimental test results showing die strength ofsilicon dies cut with 500 fs compressed pulses or 300 ps uncompressedpulses. FIG. 17 also includes published nanosecond laser results andmechanical test results for comparison. Circles are used to show resultsfor dies in tension, and squares are used to show results for dies incompression. Average values (and error bars) corresponding to theexperimental test results are offset horizontally (to the right) of theindividual experimental test results with 500 fs and 300 ps pulses.

These and other features will now be described with reference to thedrawings summarized above. The drawings and the associated descriptionsare provided to illustrate embodiments and not to limit the scope of thedisclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, target material generally refersto material in or on at least one region of a workpiece that is to bemodified by one or more laser pulses. The target material may comprisemultiple materials having different physical properties.

In the following detailed description, repetition rate, unless otherwisestated, generally refers to a rate at which laser pulses are deliveredto a target material during laser processing of the material. The ratemay correspond to the rate at which pulses are generated by a lasersource, but the rate may also be reduced relative to the source rate inembodiments where, for example, a pulse or group of pulses is gated anddelivered to the target material.

In the following detailed description, reference is made to limitingaccumulation of unwanted material within or proximate to a targetmaterial, target region, or the like. Unless otherwise stated,alternative language is to not to be construed as only one of the two(or more) alternatives, but may include both (or more) alternatives.

As used herein the term debris is not limiting, and generally refers tounwanted accumulation of material within or proximate a localizedregion. Debris may result from laser-material interaction and/or a heataffected zone (HAZ). Recast, slag, redeposit and other related terms arealso well known in the art. Typically a heat affected zone includesmaterial heated and cooled fast enough to form molten material, and theextent of the region depends, among other factors, on the pulse durationand various material parameters. Short pulses, particularly ultrashortpulses, are known to localize the heat and reduce the dimension of aheat-affected zone.

Overview

Embodiments are generally applicable for laser processing a workpiece,and particularly for micromachining applications. For example, variousembodiments are applicable to cutting, dicing, scribing, and/orengraving semiconductor substrates to form features having a typicallateral dimension of about 1 micron to about 100 microns and a depthfrom a few microns to hundreds of microns. For example, certainembodiments may be utilized for fabrication of precise trenches andgrooves in a variety of materials. Extremely precise trenches in siliconare required for a variety of microelectronic applications. Severalresearch groups have demonstrated that the best results are obtainedusing femtosecond laser pulses with the laser intensity just above theablation threshold (see, e.g., Barsch, Korber, Ostendorf, and Tonshoff,“Ablation and Cutting of Planar Silicon Devices using Femtosecond LaserPulses,” Appl. Physics A 77, pp. 237-244, (2003) and Ostendorf, Kulik,and Barsch, “Processing Thin Silicon with Ultrashort-pulsed LasersCreating an Alternative to Conventional Sawing Techniques,” Proceedingsof the ICALEO, Jacksonville, USA, October 2003)

Currently, the preferred method for micro-fluidic device fabrication isvia lithographic processing, often involving several cycles ofultraviolet (UV) light exposures that is followed by a solvent etch.Femtosecond lasers are capable of directly machining blind and throughholes of modest aspect ratio (1:10-1:100, depending upon substratematerial, laser parameters and hole diameter).

It is well known that ultrashort laser pulses offer important advantagesrelative to conventional nanosecond lasers: reduced HAZ, reducedresidual stress, less sensitivity to variations in material ablationthresholds. Furthermore, it is well established that relatively highprocessing rates can be achieved by scaling laser average power(assuming fluence on the target is greater than the material ablationthreshold) and using high speed multi-pass beam scanning. Ultrashortlaser processing is also generally recognized as a suitable approach forreduction of slag, residue, melt formations, or other unwantedby-products of laser-material interaction. However, it is also wellestablished that the use of ultrashort pulses alone cannot guaranteeimproved quality. Many early experiments were carried out in vacuumwhich simplified processing. Citations to various publications, patents,and published patent applications relating to material processing withultrashort pulses, micromachining of one or more of semiconductor,metal, or dielectric materials used in semiconductor devicemanufacturing, laser-material interaction mechanisms, and systems formicromachining may be found in the priority U.S. provisional patentapplications.

Embodiments disclosed herein may be utilized to form high aspect ratiofeatures in a material, wherein a depth to width ratio is large. Suchfeatures, sometimes referred to as grooves or trenches, may be formed bycontrollably removing material from a workpiece surface. The materialmay be removed by repeatedly scanning focused laser pulses over a targetregion with a mechanism to position the target material and/or the laserpulses relative to each other.

Some embodiments may be utilized for laser cutting of materials,particularly semiconductor substrates. Such embodiments may also includethe formation of high-aspect ratio features as part of the process. Forexample, thin wafer dicing advantageously may use clean and precise cutsto separate wafer die without damaging nearby circuitry or structures.The wafer may be diced using focused laser pulses to cut through theentire wafer, perhaps changing the depthwise position of focus duringcutting in some embodiments. Alternatively, laser pulses may form a highaspect ratio feature, for example, a narrow and deep cut ofpre-determined depth. A thin depthwise portion of remaining material isthen separated using a non-laser method. In any case, it may beadvantageous for debris and contamination to be sufficiently wellcontrolled.

FIGS. 1A-1C are schematic illustrations showing a portion of a processfor laser-based material modification. Examples of focused laser spots1000-a, 1001-a are shown with an overlap factor which may be a smallfraction of a spot diameter in some embodiments. The overlap factor maybe different than schematically shown in FIG. 1A. For example, theoverlap factor may be approximately the same from spot to spot (e.g., asschematically illustrated in FIG. 1A) or the overlap factor may differfrom spot to spot. Different laser passes may utilize different overlapfactors (and/or spot shapes, spot diameters, etc.). In variousimplementations, some adjacent spots can substantially overlap (e.g.,having an overlap factor that is a small fraction of a spot diameter) orsome adjacent spots can be spaced apart (e.g., having an overlap factorthat is approximately the same, or larger, than the spot diameter). Invarious embodiments, an overlap factor may be selected to providemachined features with, for example, smooth straight edges, or selectedto affect heat accumulation within a region. In such embodiments, theoverlap factor (or other parameters) can be pre-selected prior tomachining, selected or adjusted dynamically during machining, or acombination of pre-selection and dynamic selection can be used. AlthoughFIG. 1A illustrates the focused laser spots 1000-a and 1001-a as circleshaving the same spot diameter, the focused laser spots can have othershapes and sizes in other embodiments. Many variations in spot shape,spot size, overlap factor, etc. are possible.

The spots may be applied to target material of a workpiece with one passor with multiple passes, for example with a scanning mechanism (notshown). In FIGS. 1A-1C, the upper illustrations schematically show thefirst pass of the laser pulses (Pass1), and the lower illustrations showthe N^(th) pass of the laser pulses (Pass N). In various embodiments,any suitable number N of processing passes may be used, for example, 1,2, 5, 10, 100, 250, 700, 1000, or more passes. A simplified topschematic view of a target region is shown in FIG. 1B, showing a regionwhere material was removed with the circular spots 1000-a, 1001-a. Theregion has a lateral dimension on the order of a spot diameter, althoughit is generally known that with ultrashort pulses it is possible tocontrollably remove material over a region smaller than a spotdimension, as taught in, for example, U.S. Pat. No. 5,656,186. In thetop views shown in FIGS. 1A-1C, the region where material is removed isschematically shown as a rectangle, although at least the edgesorthogonal to the scan direction are typically somewhat rounded,particularly with the use of focused laser beams having an elliptical orcircular cross section.

With various embodiments, the features may be formed with laser spots toremove a depthwise portion of the target material, for example, about0.5 μm or a few microns in some embodiments. In a single pass, arelatively small depth-portion 1000-c is removed (see upper illustrationin FIG. 1C). A second pass, or N passes, then remove additionaldepthwise portions, as schematically represented by curve 1001-c (seelower illustration in FIG. 1C). After N passes a feature may be formedhaving a desired depth and/or spatial profile. Alternatively, with asufficiently large number of passes, the material may be cleanly severed(e.g., cut all the way through the material, sometimes called“breakthrough”). In various embodiments, the number of passes N may be1, 2, 3, 4, 5, 10, 25, 100, 250, 500, 750, 1000, 1500, 2000, 5000, ormore. The number of passes may be selected based on factors including,for example, the desired depth and/or spatial profile of the feature,the material(s) forming the workpiece, whether breakthrough is desired,and so forth. The number of passes may be dynamically adjusted duringprocessing.

FIG. 1B schematically shows a simple linear/rectangular machined patternas viewed from above the workpiece. However, machined features may becircular, elliptical, interleaved, spiral or other arbitrary shapes thatwill be formed by programming the relative positions of the laser pulsesource and target material (e.g., with a scanning mechanism, as will befurther illustrated below). Similarly, the focused spot distributionsmay be non-circular and/or may have Gaussian or non-Gaussian spotprofiles. Further, various shapes may be formed as a function of depth,for example tapered, stepped, and/or curved features wherein the widthof the feature varies with depth in a pre-determined manner, orapproximately so. High aspect ratio features may be formed alone or incombination with other features, and may be connected to a region havinga lower aspect feature, for example a large diameter hole. Manyvariations are possible with the systems and methods disclosed herein.

Some parameters of interest for embodiments of “trench digging” or otherapplications may include, for example, the shape, depth, and quality ofthe trench. However, in many applications, redeposited material,commonly called recast or slag, may be formed at or very near the edgesof the narrow trenches. The quantity of redeposited material generallyincreases with increased machining depth.

FIG. 1D schematically illustrates a cross-sectional view of a machinedfeature 1001-c having a depth (as in FIGS. 1A-1C), but havingsignificant redeposited material 1005-a. The redeposited material 1005-amay be above a surface of the workpiece and/or within the machinedfeature 1001-c. A baseline of the non-processed substrate is depicted asthe dashed lines in FIGS. 1D and 1E. The redeposited material may alsoaccumulate within a feature or target region, for example within a depthof several microns below the baseline (see FIG. 1D).

FIG. 1E is representative of an example result obtainable with pulsedlaser embodiments, wherein, for a fixed number of passes N, accumulationof redeposited material 1005-b is reduced (compared to the resultschematically shown in FIG. 1D). As illustrated in FIG. 1E, the crosssectional area of redeposited material is reduced (relative to FIG. 1D)and/or the type of material deposited is in the form of fine particlesas opposed to molten material of a larger dimension. For example, insome embodiments, such a result is obtainable by increasing the laserrepetition rate and, in this example, holding other laser parametersapproximately constant. In various embodiments, the accumulation of theredeposited material may be reduced within the target region, proximateto the target region, or both. In various embodiments, the nature of theredeposited material (e.g., the size distribution of the particles) maybe altered within the target region, proximate to the target region, orboth. FIGS. 1C, 1D, and 1E schematically illustrate the machinedfeatures 1000-c and 1001-c as having a cross-section shaped generally asa trapezoid. The trapezoidal cross-sectional shape is intended to beschematic and is not intended as a limitation on the cross-sectionalshape (or any other characteristic) of features that can be machinedwith various embodiments of the laser-based processing systems andmethods disclosed herein. In other embodiments, features can be machinedthat do not have trapezoidal cross-sectional shapes such as, forexample, triangular shapes, rectangular shapes, rounded shapes, taperedshapes converging to a minimum width much smaller than the a maximumwidth, or any other suitable shape. Many feature shapes and sizes arepossible. Also, the cross-sectional size and shape of the redepositedmaterial 1005-a, 1005-b are intended to be schematic and are notintended as a limitation on the sizes and/or shapes of possibleredeposited material.

By way of example, results from machining experiments on siliconsubstrates showed a surprising result: increasing the laser repetitionrate of laser pulses from about 200 kHz to about 1 MHz, whilemaintaining approximately constant laser pulse energy, focal spot size,and pulse duration, produced an increase in the volume of materialremoved relative to the amount of material redeposited. The experimentswere carried out using a fiber-based ultra-short chirped pulse lasersystem. The results suggest that pulse repetition rates of severalhundred kHz up to several MHz may provide a significant improvement inprocessing quality. For example, in certain applications, additionalprocessing steps may not be required to remove redeposited material.

Obtaining both a desired feature shape and reduction in redepositedmaterial were best achieved with ultrashort pulses, for example, pulsesless than about 10 ps in width. However, increased repetition rate wasalso beneficial with longer pulses of about 200 ps. The accumulation ofredeposited material was reduced relative to slower repetition rates.For some applications, benefits may also be found with longer pulsewidths, for example up to a few nanoseconds, or below 10 ns.

Embodiments may therefore decrease the quantity of slag and/or otherunwanted material (and/or change the nature of the redepositedmaterial), while providing for a desired shape, depth, and/or width ofthe features. By way of example, and as will be shown later, highrepetition rate processing affected the nature and quantity ofre-deposited material.

In certain embodiments, a measure of quality may be the depth and/orvolume of a machined feature relative to a peak height, average height,and/or volume of redeposited material. Another example measure ofquality may be the feature depth relative to the total volume ofredeposited material. Suitable measures of quality may be obtained withcross-sectional samples or volumetric quantification of an affectedregion. Various tools may be used to quantify performance, for example,surface metrology tools such as white light interferometers, AtomicForce Microscopes (AFMs), or similar tools (available from, for example,Veeco Instruments Inc., Woodbury, N.Y.). The tools may provide forimproved measurement accuracy and precision, with capability forautomated or semi-automated operation. The commercially available toolshave proven capability for measuring surface roughness of a sample andalso larger volumes of material, and AFMs may be used to quantifystructure of the depthwise features, for example.

In some applications, for example dicing and scribing, different qualitymeasures may be provided. For example, quantification of the volume ofredeposited material may be a useful measure, and may be combined withcut quality as an overall figure of merit. Various embodiments areparticularly applicable for processing operations where high efficiencyis desirable, and wherein accumulation of redeposited material isdetrimental or otherwise undesirable.

In some embodiments micromachining may include laser scribing, dicing,or similar processing of semiconductor wafers, which may be bare orpatterned. Scribing and dicing are two applications with a recognizedneed. Scribing typically removes one or more layers of multiplematerials supported on a silicon substrate. The die of a wafer may thenbe separated with a mechanical dicer. With decreasing of siliconsubstrate thickness to below 100 μm, for example 50 μm, an increasedneed for laser based dicing of the substrates has developed. However, insome implementations, rapid laser processing speeds are required toprovide justification to replace conventional mechanical dicing.Moreover, in some implementations, undesirable thermal effects are to bereduced or avoided to assure reliability of subsequent packagingprocesses.

FIGS. 1G-1 (not to scale) schematically illustrates an example of apatterned semiconductor wafer 120 having several die arranged in rowsand columns with streets 127 therebetween. In conventional systems thewafer is typically laser scribed, and cut using a dicing saw. As thethickness decreases below about 100 μm, for example 50 μm or 75 μm,mechanical dicing becomes more difficult. Therefore, it is desirable touse laser dicing to reduce or eliminate mechanical dicing.

FIG. 1G-2 schematically illustrates an example portion 125 of the wafer120. By way of example, dicing is to be carried out in region 127-Balong the streets. The region may include several materials and barewafer portions. The circuit features shown in the streets, for examplehigh-density grid layer 129, may be utilized for electrical or otherfunctional tests prior to dicing. The regions adjacent to street 127contain high density active circuits, interconnects which may includesolder balls, or other combinations. In certain advantageousimplementations, the dicing or scribing is to be carried out to cleanlycut the wafer without causing damage to circuitry, without introducingsignificant debris or heat affected zone (HAZ), and should provide forsufficient die strength.

FIG. 1G-3 schematically illustrates a cross sectional side-view 129-1 ofa portion of the wafer, the fine grid area 129 of FIG. 1G-2. The gridmay be covered with one or more of dielectric and metal materials.

Because potential processing speed is one possible reason for use oflaser technology for thin-wafer dicing, a practical system for dicingvery thin wafers is to provide for removal of a relatively large amountof material at high speed.

Workpiece materials in the streets may include, but are not limited to,metals, inorganic dielectrics, organic dielectrics, semiconductormaterials, low-k dielectric materials, or combinations thereof. Thecombinations of materials may be arranged in widely varying spatialpatterns and/or stacked in depth. For example, microelectronic circuitsmay comprise portions having alternating layers of copper and low-kmaterial, covered by an overlying passivation layer. Other combinationsand/or configurations of materials are possible.

Various studies disclosed results and models for micromachining ofSilicon. For example, Crawford et al, in “Femtosecond laser machining ofgrooves in Silicon with 800 nm pulses, Applied Physics A 80, 1717-1724(2005) investigated ablation rates (in vacuum) as a function of pulseenergy, translation speed, number of passes, and polarization direction(parallel vs. perpendicular to translation direction, and with circularpolarization). Laser parameters included 150 fs pulses at 800 nmwavelength with laser pulse repetition rate of 1 KHz. Maximumtranslation speed was about 500 μm/sec. A spot size was about 5 μm.

Single and multiple pass results were reported, and motion effectsanalyzed. A model was disclosed, including the effects of motion withhigh overlap between pulses assumed. The approach included determiningan accumulated fluence at a point along the center of a groove. Despiteproviding a useful framework for analysis, it was recognized that theeffective fluence may change somewhat for each pulse and a single or fewpulse irradiation may produce much different results than many pulses,whether or not the target is moving. Some conclusions reflect a littleeffect of translation on groove width, with the effect being difficultto quantify due to roughness and debris. The results were also comparedwith other studies. Various other morphologies were identified.

Ablation performance was not predictable with a linear model over allpasses. Reported ablation depth per pulse well below 1 μm were generallyobserved with fluencies up to a few Joules/cm². Polarization effectswere somewhat significant, with branching with polarization parallel totranslation direction. Expected ablation depth limits were observed withgroove formation, apparently a result of insufficient fluence at thebottom of a groove. In one example, the first few passes resulted inlarge amounts of material removed in a nearly linearly manner. However,beyond twenty passes the amount of material ejected decreased. Materialre-deposition apparently competed with removal by the edges of the pulsenear the rim of a groove. After a large number of passes the rim wasexpected to largely erode away with additional passes.

With our experiments increasing the repetition rate to about a fewhundred KHz or greater, and preferably to at least about 1 MHz in someembodiments, improved a ratio of material removed to redepositedmaterial compared to results obtained below a few hundred KHz. Theresults were obtained with translation speeds suitable forhigh-throughput processing, and approaching some present limits ofmotion speed of high speed mirror systems. Also, at least some resultsindicate too high a repetition rate will result in undesirable thermaleffects, recast, and generally unwanted HAZ induced materialmodification. Simultaneously achieving both high throughput and reduceddebris is a general goal and a beneficial result that can be achievedwith certain embodiments.

By way of example, scribing and/or dicing of 50 μm thick or similarsubstrates may be carried out with a focused spot size of at least 15μm, and with a spot size in the range of about 15-50 μm in someembodiments. Other spot sizes may be used such as, for example, a fewmicrons (e.g., about 3 microns in one case). In some implementations,spot sizes in a range from about 1 micron to about 5 microns are used.The quantity of material removed is generally determined by one or morefactors including the scan speed, spot overlap, repetition rate (pulsesper second delivered to surface), pulse energy, and spot diameter. Insome embodiments, sufficient overlap between adjacent spots on thesurface provides for cutting or scribing a pattern of relatively uniformwidth. In some experimental systems, relatively high pulse energy of atleast a few microJoules with a spot size of about 15 μm will typicallyresult in ablation within a region having diameter about 15-20 μm. Ascanning system, for example a galvanometer based mirror scanner, mayprovide scan speeds of up to about 10-20 msec.

Material removal requirements vary, and heat accumulation within aregion may be increased or decreased with suitable selection of one ormore factors including pulse energy, repetition rate and speedparameters. It may be desirable to increase heat accumulation within aregion to facilitate material removal in some embodiments. In variousembodiments ultrashort pulses may be applied at a high rate and reducedmotion speed to induce thermal effects similar to non-ultrashort pulses.In at least one embodiment one or more of pulse energy, repetition rate,speed, and pulse width may be adjusted. In some embodiments availablepulse energy will be at least about 5 μJ, repetition rates will beadjustable up to about 10 MHz, beam speed at the surface may be in arange of about 0.1 m/sec up to about 10 msec, and pulse widths providedwithin a range from below 1 picosecond up to a few nanoseconds. By wayof example, with 1 MHz rate, 40 μm spots, and speed of 0.1 msec, theoverlap between spots exceeds 99%. Localized heat accumulation may besignificant. If the 1 MHz rate is maintained, and speed is increased toabout 5 m/sec, the spot overlap decreases by 50-fold, with decreasedheat accumulation within a processing region. Accordingly, in variousadvantageous embodiments, overlap factors may be utilized that are in arange from about 0.001 to about 0.99. Other ranges are possible.

Because materials within the streets may vary with different waferdesigns it is desirable for some implementations of a laser system toprovide for adjustment of certain parameters over a wide range. Forexample, the scan speed, pulse energy, repetition rate (rate at whichpulses impinge the surface), pulse width, and spot size are preferablyadjustable over a wide range, for example at least 2:1 in someembodiments. One or more such parameters, for example the pulse width,speed, and repetition rate, may be adjustable over more than a 10:1range. Other adjustable ranges are possible in other embodiments.

Different laser and speed parameters may be required for scribing andbare silicon dicing, as a result of different material properties. Insome embodiments material removal will be facilitated with increasingheat accumulation with relatively high pulse energy and high overlapbetween pulses, for example greater than 99% overlap. A sufficientlywell controlled heat-affected zone (HAZ) is to be maintained to avoidcollateral damage or increased debris in some of these embodiments.

A typical multimaterial device, for example a patterned wafer, mayinclude conductor, dielectric, and semiconductor materials stacked indepth. Processing of a typical multimaterial device may be carried outat, for example, a 1 MHz repetition rate, a spot size of about 40 μm,and at motion speeds producing overlap between about 75% to more than99% between adjacent spots.

By way of example, with a pulse repetition rate in the range of about afew hundred kHz to about 10 MHz, the scan speed may be controlled insuch a way that tailors the heat accumulation to facilitate materialremoval while simultaneously limiting debris and controlling HAZ. Insome implementations, removal of metal and dielectric layers may becarried out at a scan speed substantially slower than a scan speed usedfor removal of bare silicon. As a result, the overlap between adjacentspots for removal of at least one of a metal and dielectric may begreater than an overlap for bare wafer processing (e.g., at least aboutten-times greater in some embodiments). A focused spot size in the rangeof about 15-40 μm, and typically about 30-40 μm, provides for highthroughput in some cases.

Referring again to FIG. 1G-2, the width of street 127 may be reduced incertain wafer designs, for example to a few ten of microns. Acorresponding reduction in spot size from certain preferred values above(e.g.: about 40 μm in some cases) may be advantageous. For example, aspot size of about 5 μm may be useable for cutting, scribing, or otherprocessing operations within a street having a width of about 25 μm.Some laser parameters may be scaled accordingly, and various designoptions may be utilized to avoid physical limitations associated withcertain parameters. Other spot sizes may be used such as, for example, afew microns (e.g., about 3 microns in one case). In someimplementations, spot sizes in a range from about 1 micron to about 5microns are used. Spot sizes of about a few microns may be advantageousfor processing narrow street widths (e.g., less than a few tens ofmicrons).

Referring again to FIG. 1G-2, a cutting path 127-b within street 127 isillustrated, the cutting path 127-b centered on the street region inthis example. It is known that wafer scribing and breaking may becarried out with a combination of a laser, for example a nanosecondpulsed laser, and a dicing saw. The nanosecond laser may, in someimplementations, scribe two lines at approximately equal distance fromthe center of the street. A dicing blade centered on the street is usedto cut through the remaining wafer, thereby producing individual die.Embodiments described herein may also be used to modify material alongany path (e.g., a pre-determined path for the wafer), and/or may be usedin various combinations (e.g., with a dicing saw). The material to bemodified or processed may comprise metal, dielectrics including low-kmaterials, and/or semiconductors. Moreover, processing of ultra-thinwafers, such as, for example 50 μm thick wafers, can be carried out witha femtosecond laser to cut through the entire thickness of the wafer, ora substantial portion thereof, in some implementations. Some embodimentsmay reduce or eliminate use of mechanical dicing of such ultra-thinwafers.

For example, in certain implementations, requirements for precisionpositioning may increase, but total pulse energy may be decreased. It iswell known that a decreasing spot size at a particular wavelengthresults in a decreased depth of focus (DOF). The DOF decrease generallyvaries as the square of the spot size. If processing over a large depthrange is required various well known methods and systems for dynamicfocusing, or improvements thereof, may be applied in some cases. By wayof example, as the spot size decreases from 50 μm to 5 μm the DOFdecreases by 100-fold. On other hand, the total pulse energy to achievea given fluence over a spot area decreases as the square of the spotdiameter. In some implementations, much lower maximum pulse energy maybe used for smaller spot sizes, and a maximum pulse energy may be, forexample, about 100 nJ, or up to about 1 μJ, for processing of variousdielectric, conductor, and semiconductor materials. A smaller spot sizemay lead to some considerations for motion control also. Reduced scanspeeds may be utilized in some embodiments, but a requirement forprecision positioning may also be increased.

Therefore, in some embodiments, pulse energy and speed may be scaleddownward while processing at a given fluence and repetition rate. By wayof example, assume pulse overlap exceeding 99% (e.g.: at least 99.5%), a1 MHz repetition rate, and a spot size of about 4 μm (e.g.: approximate10-fold reduction from the 40 μm spot size used in some embodiments). Inthis example, corresponding scan speed is on the order of 10 mm/sec. Thefluence may be obtained with pulse energy may be scaled down from arange of at least a few microjoules (e.g.: 5 μJ) to below 100 nanojoules(e.g.: 50 nJ) as a result of a 10-fold decreased spot size.

Similarly, in some embodiments, the repetition rate may be increased totens of MHz, and with relatively low pulse energy for certainmicromachining operations. For example, some cutting or scribingapplications may require selective removal of a single layer ofmaterial, or a few layers, with relatively low fluence.

Referring to FIG. 1G-3, one of more layers are schematically representedby shaded regions (not necessarily to scale), and may comprisedielectric and/or metal materials. The underlying bare wafer (notshaded) is processed by the laser after modification of the overlyinglayers in certain processing applications. The inventors also discoveredthat thermal processing (e.g., heat accumulation) and/or a sufficientheat affected zone (HAZ) reduces or avoids delamination and/or crackingof composite layers or certain material (e.g. low-k dielectrics). Also,reduced HAZ associated with ultrashort pulses may be beneficial forcutting through the wafer for singulation of die. By way of example, ifnanosecond pulses are used both to remove layers and for dicing thesilicon wafer, performance may be insufficient or unpredictable. Forexample, it is known that weak die strength and various other materialissues are caused by excessive HAZ caused by nanosecond pulses.

Without subscribing to any particular theory, when a wafer is irradiatedwith a laser pulse, electrons in the wafer absorb energy from the laserbeam almost immediately. As a result of collisions between hot electronsand the lattice, thermal equilibrium between the electron system and thelattice is quickly achieved, and the exposed area increases intemperature. The time to reach the equilibrium varies as a function ofmaterial, and may be hundreds of femtoseconds to tens of picoseconds.Thermal energy within the exposed region will transfer to itssurrounding cooler area. The rate of cooling is affected by severalparameters, for example: material, temperature differential between thehotter and cooler area, as well as the temperature distribution. As anexample point of reference, an approximate period, when silicon staysabove its melting temperature, is about hundreds of ps.

When a nanosecond (or longer pulse duration) laser is used for dicing orscribing process, the irradiated region remains above its meltingtemperature for an extended time frame. A “melting pool” (e.g.: a regionof molten material) will be formed and will shrink when it is coolingdown. The “boiling” and “cooling” process causes cracking, surfaceroughness and voids in the HAZ. Such a process can be erratic, and thequality of material modification difficult to predict.

Femtosecond pulse irradiation in some implementations provides a shallowHAZ, but little interaction with underlying layers of a device occurs asa result of the ultrashort pulse width. The femtosecond heating processis almost instantaneous, confines HAZ to a limited thickness, and doesnot substantially affect layers disposed below modified material. Muchsmoother and predictable surface morphology is achievable in certainsuch implementations.

However, as a result of the very shallow HAZ formed by an ultrashortlaser pulses in some of these implementations, little or no materialmodification of multiple materials occurs. For example, melting betweenlayers may be absent. Thus, dicing or scribing performance withmultilayer devices, specifically devices having at least one low-kmaterial, can be somewhat limited using femtosecond laser pulses inthese implementations. Moreover, delamination may occur in some of theseimplementations. However, the inventors discovered, as will be shown inexperiments described below, that in some embodiments of the systems andmethods described herein, increasing pulse energy and/or fluence, and/ordecreasing scan speed, provided good processing results inmulti-material target regions. Accordingly, the inventors' resultsdescribed herein may be used to control heat accumulation and/or HAZwithin one or more materials of the target. For example, embodiments ofthe systems and methods disclosed herein may be configured to providesufficiently high heat accumulation in a target to reduce or avoiddelamination (e.g., delamination of a dielectric material and a metalmaterial).

In some embodiments multiple lasers may be utilized, and configured inan integrated laser system having multiple sources, or as a sourcehaving adjustable pulse widths. By way of example, a relatively longpulse width, for example hundreds of picoseconds and up to a fewnanoseconds, may be utilized to increase HAZ for processing a firstdepthwise portion of a workpiece, and particularly for removing low-klayers and/or other metals and/or other dielectrics. Such metals mayinclude, but are not limited to, copper, aluminum, and/or gold.Dielectric materials may include, but are not limited to, silicondioxide, silicon nitride, and/or various organic or inorganic materials.The arrangement of the dielectric and/or metal materials may vary inthree-dimensions as schematically illustrated in the examples shown inFIGS. 1G-1 to 1G-3.

In some embodiments, ultrashort pulses may be used to process a seconddepthwise portion of the workpiece, with generation of negligible HAZ.In various embodiments femtosecond pulses are utilized for at leastcutting through the entire wafer, or a substantial fraction of thewafer, and particularly for cutting very thin wafers, for example wafershaving thickness of 100 μm or less. Moreover, in some of theseembodiments, at least a portion of the processing of metals and/ordielectrics may also be carried out with femtosecond pulses.

In at least one embodiment a single laser source may be utilized.Adjustment of laser parameters may balance heat generation within aprocessing region and transfer of heat outward from the region.Delamination and/or unwanted thermal stress are then reduced or avoided.

In various embodiments a picosecond pulse width may be utilized toremove a low-k material. For example, at least one pulse may be in therange of about 100 ps to about 500 ps, about 100 ps to 250 ps, or in therange of about 200 ps to 500 ps. In some embodiments at least one pulsemay have a pulse energy may in the range of about 2 μJ to 10 μJ over aspot diameter of 30-40 μm, corresponding to a fluence at least about0.15 J/cm², for example. Such examples of pulse widths and fluence cangenerate sufficient HAZ for processing metal and dielectrics, and withina period of time to provide material modification, (e.g.: melting andremoval) of multiple layers. However, any region of melted material isalso sufficiently shallow (e.g.: not too deep) so that unwantedcracking, surface roughness and/or voids in the HAZ are reduced oravoided. In other embodiments, other pulse widths, pulse energies, spotdiameters, and fluences may be used.

In some device designs the width of street 127 may be reduced.Embodiments of the laser system may then be configured with a reducedspot size for processing in a narrowed region. In some of theseembodiments, the pulse energy can then be reduced while maintaining agiven fluence. However, in some applications a relatively high fluencemay be selected for processing and may be advantageous for processing ofvarious metals and dielectrics.

Example Embodiments of Pulsed Laser Systems for Micromachining

FIG. 1F schematically illustrates an embodiment of a system 100 suitablefor processing a workpiece with laser pulses. The system 100 comprises alaser system 104 that is operatively coupled to a controller 114 andscanning system 106. In some embodiments, the laser system 104 isconfigured to output laser pulses that comprise one or more ultrashortpulses (USP). For example, in at least one embodiment, the laser system104 comprises a USP laser. In various embodiments the system 100 willprovide for adjustment of certain pulse parameters over a substantialrange. Such parameters may include one or more of pulse energy, pulserepetition rate, pulse width, spot diameter, overlap of adjacent spots,and scan speed. By way of example, pulses may be generated at anadjustable repetition rate up to about 1 MHz, or up to about 10 MHz. Anoutput pulse may have an energy of about 1 μJ or higher, for example upto about 5-20 μJ, and a pulse width about 1 ps or shorter. Furtherdetails of various embodiments of the system 100 are described below.

An amplified laser system, particularly an ultrashort fiber-basedchirped pulse amplification system (FCPA), operating at repetition ratesof at least several hundred kHz, is suitable for processing of severaltypes of patterned and unpatterned substrates. High pulse energy, forexample several microjoules, is obtainable with an amplified train ofultrashort pulses. Sufficient pulse energy in at least the microjoulerange is obtainable, with 15-40 μm typical spot diameters providing forhigh throughput in some embodiments.

In some implementations, multiple passes can be used. The pulse energyused in the passes may be the same or different than the energy used inadditional passes. Moreover, in some embodiments, the pulse energy maybe varied between passes.

In some embodiments other laser pulse parameters may be adjusted betweenpasses. For example, a relatively long pulse width may be used forremoval of at least conductive and/or dielectric materials. Such a pulsewidth may be up to a few nanoseconds (ns), less than 1 ns, or about 500ps or shorter. An ultrashort pulse may be used to cut at least theunderlying silicon material, for example with sub-picosecond pulses.

In some implementations, the long and short pulses may be applied inseparate passes, or in some embodiments by applying bursts of laserlight to a target area of a material during any single pass. In somecases, the burst may be applied at a predetermined repetition rate, andmay comprise at least first and second pulses of laser light displacedor overlapped in time, and the first pulse width may be greater than thesecond pulse width, and greater than 10 ps in duration in someembodiments, the second pulse width being an ultrashort pulse, forexample a sub-picosecond pulse. The pulse separation of pulses in theburst may be about 1 μsec to 0.1 μsec, and in some embodiments a shorterseparation may be used. The second pulse width may be as above:sub-picosecond (e.g.: >100 fs) to about 10 ps, and generally less thanabout 50 ps. Moreover, first and second is not restricted to temporalsequence, but may be applied in any order. For example a reversed ordermay result from respective top-side or bottom-side initial scans.

One possible preferred laser system for some micromachiningimplementations will provide pulse energy of at least about 5 μJ at anadjustable repetition rate (pulses delivered to the surface) of about afew hundred kHz to 10 MHz, and will be coupled to a scanner for scanningat a rate up to about 10 msec. The system can include an optical poweramplifier to provide for high pulse energy and sufficiently highthroughput. Preferably at least a portion of the system will be fiberbased.

In one preferred embodiment, the laser source comprises a Yb-doped,amplified fiber laser (e.g., FCPA μJewel, available from IMRA America).Such a laser offers several primary advantages over commercialsolid-state laser systems. For example, this laser source provides avariable repetition rate over a range of about 100 kHz to 5 MHz. Higherpulse energy than oscillator-only systems allows greater flexibility infocal geometry (e.g.: larger spot sizes for a given fluence). In atleast one embodiment, pulse energy of up to about 10 μJ may be appliedat a repetition rate of about 1 MHz, with at least about 1 μJ at a 5 MHzrate. Higher repetition rate than various solid-state regenerativelyamplified systems allow greater speed. Although some oscillators havebeen demonstrated which produce microjoule pulse energy, the complexityis at least comparable to CPA systems.

Such energy is also achievable with embodiments of a fiber-based systemutilizing a power amplifier, for example at least one large modeamplifier producing a nearly diffraction limited output beam. In atleast one embodiment, a large mode amplifier may receive low-energypulses from a mode locked fiber oscillator, and amplify the pulses tothe microjoule level. Preferably, the oscillator and power amplifier areintegrated to form an all-fiber system. Numerous possibilities exist.

In some embodiments, particularly for processing with lower pulse energyand/or higher repetition rates, an all-fiber ultrashort pulsed lasersystem may be utilized. The system may include a fiber-based pulseamplification system producing pulse widths below 1 ps. Low energypulses from a fiber oscillator may be selected with an optical switch,and amplified with a fiber amplifier to at least about 100 nJ. Atrelatively low energy the sub-picosecond pulses may be amplified withthe fiber amplifier. In other embodiments an all-fiber chirped pulseamplification system may comprise a pulse stretcher and pulsecompressor. The compressor may comprise a fiber compressor performing atleast partial pulse compression, a bulk compressor, or a combinationthereof. Many variations are possible, including further amplification,harmonic conversion, and the like.

Various embodiments include fiber-based chirped pulse amplificationsystems suitable for numerous micromachining applications. The systemsare particularly suited for processing materials using pulse energies upto tens of microjoules and up to a maximum of about 100 μJ. Spotdiameters may be in a range from about 1 micron to about 100 μm. In someembodiments, a spot size may be in the range of about 10 μm to 100 μm,or 10 μm to about 60 μm, or 25 μm to 50 μm. Pulse widths may be in arange from tens of femtoseconds (e.g., 50 fs) to about 500 picoseconds.The parameters generally provide for energy density near or above anablation threshold for the workpiece material(s) being processed, andthe total energy required may depend on, for example, the spot diameter.Workpiece materials may include, but are not limited to, metals,inorganic dielectrics, organic dielectrics, semiconductor materials,low-k dielectric materials, or combinations thereof.

FIG. 1F schematically illustrates a first embodiment of a system 100capable of use for processing a workpiece, for example a semiconductorsubstrate. The system 100 comprises a laser system 104 and a scanningsystem 106. In this embodiment, the scanning system 106 includes twobeam deflectors 108, for example galvanometric scanning mirrors, capableof two-dimensional scanning. In other embodiments, a different numberand/or type of scanning mirrors may be used. In some embodiments, thescanning may be one-dimensional. The scanning system 106 may alsoinclude focusing optics 110 such as, for example, an integrated F-thetalens capable of producing a substantially flat field of view at thetarget substrate 112. For example, in some embodiments, the F-theta lensis configured to produce a 20 μm laser focus spot with a substantiallyflat field of view over an area of about 8000 mm². In other embodiments,for example for application to wafer cutting or dicing, a 10-50 μm laserfocus spot with a substantially flat field of view over an area of about60 mm×60 mm may be utilized. The scanning system 106 (and/or othersystem components) may be controlled by a controller 114. For example,the controller 114 may include one or more general and/or specialpurpose computers, which may be remote and/or local to the system 100.

In other embodiments, additional optical elements may be utilized in thescanning system 106 (e.g., mirrors, lenses, gratings, spatial lightmodulators, etc.). A skilled artisan will recognize that a pattern to beformed within the substrate may be communicated to the system 100 viamany methods including wired and/or wireless techniques. In certainembodiments, the pattern is represented via vector graphics includingcurves and/or polygons, and may include three-dimensional machininginstructions. Many variations are possible.

In some embodiments, the laser system 104 may comprise a USP laserconfigured to output one or more ultrashort pulses (USP). An ultrashortpulse may have a duration such as, for example, less than approximately10 ps. In the example system 100 shown in FIG. 1F, the laser system 104may comprise a fiber-based laser capable of generating an ultrafastpulse train. For example, the laser may comprise an FCPA μJewel laseravailable from IMRA America, Inc. (Ann Arbor, Mich.). The laser pulseshave a wavelength that may be about 1 μm. In some embodiments, shorterwavelengths laser pulses are used such as, for example, green lightpulses of about 520 nm wavelength. In other embodiments, any othersuitable laser system can be implemented. In certain embodiments, thelaser system 104 may produce laser pulses with a pulse width less thanabout 10 ps. For example, the pulse width may be in a range from about100 fs to about 1 ps. In some embodiments, the pulse width is in a rangefrom about 10 fs to about 500 ps. In other embodiments of the lasersystem 104, other pulse widths are used such as, for example, ≦10 ns, ≦1ns, ≦100 ps, ≦1 ps, and/or ≦100 fs.

In certain embodiments, the laser system 104 may comprise a diode-basedand/or microchip laser seed source and may output pulses havingdurations of about a nanosecond, a few nanoseconds, and/or up to about10 nanoseconds. The laser system 104 may comprise any suitable type oflaser for outputting pulses having desired properties.

In some embodiments, a relatively high laser repetition rate is used toenable relatively rapid laser processing. For example, the repetitionrate may be larger than 500 kHz. In certain embodiments, a repetitionrate of about 1 MHz to 10 MHz may be used. Other repetition rates arepossible. Based on results disclosed herein, the use of a relativelyhigh repetition rate may be utilized in some embodiments to reduce thequantity of redeposited material 1005-a schematically illustrated inFIG. 1D. In some implementations, tens or hundreds of laser pulses mayoverlap in each focal spot diameter, which may be about 20 μm indiameter, or 10-50 μm in some embodiments. In other embodiments adifferent number of pulses may overlap. For example, in some embodimentsa few pulses may overlap, for example 3 pulses. Another possibleadvantage of a relatively high repetition rate is the ability to processthe substrate in a shorter time than when a lower repetition rate isused. As such, in certain embodiments, the throughput of the system 100is improved while simultaneously providing improved quality.

FIG. 2A schematically illustrates an embodiment of a system 200 that canbe used for processing a semiconductor a target substrate 112 via withultrafast pulse trains. This system 200 may be generally similar to theembodiment schematically depicted in FIG. 1F. The laser system 104 inthe embodiment shown in FIG. 2A comprises an optional internal pulsemodulator 202 not shown in the embodiment depicted in FIG. 1F. Theoptical modulator 202 may be used for modulation of the repetition rateof the laser pulse train. In some embodiments, the laser pulse traincomprises one or more ultrashort pulses such as, for example, one ormore trains of ultrashort pulses. In some embodiments, the modulator 202is adapted to change the laser pulse repetition rate from the oscillatorrepetition rate (typically about 50 MHz in some fiber laser embodiments)to the machining repetition rate (typically less than or about 1 MHz).For example, the modulator 202 may be configured to allow fortransmission of only every nth pulse from the oscillator pulse train toa final power amplifier, or transmission of groups of pulses. In certainembodiments, it may be convenient to implement such oscillator amplifierconfigurations for the generation of high energy pulse trains, where forimproved oscillator stability, oscillator repetition rates of the orderof 50 MHz are utilized. Such oscillator amplifier systems are well knownto a skilled artisan.

In certain implementations, the internal modulator 202 allows theaverage power and thermal conditions in the amplifier to remainsubstantially the same while substantially instantaneously changing thepulse energy and pulse peak power. The internal modulator 202 maycomprise an acousto-optic modulator or any other suitable opticalmodulator. In certain embodiments, the laser system 104 outputs pulseswith pulse energies above about 1 μJ, pulse durations less than about 10ps, and a pulse repetition rate of greater than about 100 kHz.

The embodiment shown in FIG. 2A also comprises a frequency converter 204such as, for example, a second harmonic generation (SHG) converter. Inthis embodiment, combination of the SHG converter and the internalmodulator 202 provides a “fast shutter,” because the harmonic conversionefficiency is proportional to the laser pulse energy. Accordingly, bymodulating the laser repetition rate from the oscillator it is possibleto turn the machining beam (e.g., the transmitted SHG beam) on and offsubstantially instantaneously. Such rapid shuttering is not possiblemechanically and is difficult to implement optically for high laserpowers without causing degradation to beam quality, pulse duration, etc.Some embodiments may include a third harmonic generation converterand/or a fourth harmonic generation converter or any other suitableharmonic generation converter.

The embodiment shown in FIG. 2A also comprises the controller 114, whichmay be used to control the laser system 104, the scanning system 106,the frequency converter 204, and/or other system components. Forexample, in certain embodiments, control of the modulator 202 and thescanning system 106 (e.g., the scanning mirrors 108 and/or the focusingoptics 110) may be linked so as to enable much greater control of thelaser irradiation conditions, thereby providing greater control ofmachining depth and lateral extent. For example, in some embodiments,the controller 114 is configured to control a spatial overlap betweenadjacent focused pulses (or groups of pulses) during processing of atarget material at the pulse repetition rate.

FIG. 2B schematically illustrates an embodiment of a system 230 capableof use for processing target substrates with ultrafast pulse trains. Inthis embodiment, the laser system 104 includes a chirped pulseamplification system such as, for example, a fiber-based chirped pulseamplification (FCPA) system. Advantages of using an FCPA system includeimproved efficiency and reliability. Also, since the output energy andpeak-power of a fiber amplifier generally decrease as the repetitionrate of the oscillator increases, with substantially constant averageoutput power or with fixed pump power. The fiber amplifier output energyand power variation as a function of repetition rate may be exploited toprovide improved FCPA performance.

Various U.S. patents assigned to the assignee of the present applicationdisclose chirped pulse amplification systems using compact fiberconfigurations. The disclosure of each of the following U.S. patents ishereby incorporated by reference herein in its entirety: U.S. Pat. No.5,499,134, issued Mar. 12, 1996 to Galvanauskas, et al., entitled“Optical Pulse Amplification Using Chirped Bragg Gratings,” U.S. Pat.No. 5,696,782, issued Dec. 9, 1997 to Harter, et al., entitled “HighPower Fiber Chirped Pulse Amplification Systems Based On Cladding PumpedRare-Earth Doped Fibers,” and U.S. Pat. No. 7,113,327, issued Sep. 26,2006 to Gu, et al., entitled “High Power Fiber Chirped PulseAmplification System Utilizing Telecom-Type Components” (hereinafterreferred to as “the '327 patent”). Any of the laser systems disclosed inthese patents, as well as other commercially-available “all fiber” lasersystems, may be used with the system 230 shown in FIG. 2B.

In certain embodiments, the laser system 104 comprises an FCPA μJewellaser (available from IMRA America, Inc., the assignee of the presentapplication), which provides laser pulses at an output of a compressor252. The output pulses may be generated at an adjustable repetition rateup to about 1 MHz. An output pulse may have an energy of about 1 μJ orhigher, and a pulse width about 1 ps or shorter. In some embodiments, ifthe peak power and pulse energy are low enough to avoid non-lineareffects, a fiber compressor, rather than a bulk output compressor, maybe used for pulse compression. In certain embodiments, photonic bandgapfibers or photonic crystal fibers may be used alone or in combinationwith bulk compressors or large area fibers to provide for increasedoutput energy and peak power.

In the embodiment of the system 230 schematically illustrated in FIG.2B, the laser system 104 comprises a single-pass fiber-based chirpedpulse amplification system. The laser system 104 includes a highrepetition rate source 232, a fiber stretcher 236, a fiber pre-amplifier240, a pulse selector/modulator 244, a fiber power amplifier 248, and acompressor 252. The output of the compressor 252 may be an ultrashortpulse train. In some embodiments, the compressor 252 may be detuned toprovide longer pulse widths (e.g., about 200 ps). In other embodiments,the compressor 252 is not used, and the laser system 104 outputs pulseshaving widths up to about a nanosecond, a few nanoseconds, and/or up toabout 10 nanoseconds. In some embodiments, the laser system 104 mayinclude one of more of a single-pass and double-pass pre-amplifier, asingle or double-pass stretcher, and power-amplifier arrangement (notshown), which may provide longer stretched pulse widths and higher pulseenergy in a comparable package size. Some embodiments may comprisepolarization maintaining (PM) fiber amplifiers, oscillators, andstretcher fibers. As described above, the controller 114 may beconfigured to coordinate delivery of the pulses to the target substrate112 via the scanning system 106. In various embodiments, the controller114 may be used to control some or all of the components of the lasersystem 104, the scanning system 106, and/or other system components. Inone embodiment, the controller 114 is configured to control the lasersystem 104 by controlling the pulse selector/modulator 244. As describedabove, the scanning system 106 may include, for example, a scanningmirror 108 such as, e.g., a galvanometer scanning mirror. The scanningsystem 106 may also include focusing optics 110.

The high repetition rate source 232 may provide a free-running pulsetrain operating at a repetition rate well above 1 MHz, for example, in arange of about 20 MHz to about 100 MHz. Mode-locked lasers, includingall-fiber-based passive mode-locked or other devices, may be used toproduce such repetition rates. Corresponding pulse widths may be in arange from about several hundred femtoseconds to about 10 picoseconds,for example. In other embodiments, non-mode locked laser sources may beused. For example, output of a quasi-cw semiconductor laser may bemodulated and optionally compressed to produce picosecond or femtosecondpulses. Suitable laser sources include the sources described in U.S.patent application Ser. No. 10/437,057 to Harter, entitled “InexpensiveVariable Rep-Rate Source For High-Energy, Ultrafast Lasers,” now U.S.Patent Application Publication 2004/0240037, assigned to the assignee ofthe present application, and hereby incorporated by reference herein inits entirety.

The fiber stretcher 236 may include a length of optical fiber (e.g.,about 100 m to 1 km depending on fiber dispersion) to stretch pulsesfrom the high repetition rate source 232 in order to avoid non-lineareffects and/or damage to the fiber pre-amplifier 240 and/or the fiberpower amplifier 248. The stretcher 236 may comprise a fiber Bragggrating (FBG), a chirped FBG, or a combination thereof. The stretcher236 may comprise fiber having anomalous third order dispersion (TOD), soas to partially compensate residual TOD (if present) that may beaccumulated in the system. In some embodiments, the majority of residualTOD results from the use of mismatched stretcher (fiber-based) andcompressor dispersion (bulk-grating based). In various exampleembodiments, the width of a stretched pulse may be about 50 ps, in arange from about 100 ps to about 500 ps, or in a range up to about 1 ns.Pulse stretching may also be provided in double pass arrangements.

The fiber pre-amplifier 240, which is optional in some laser systems104, amplifies the energy of a pulse emitted from the high repetitionrate source 232. The source 232 may emit pulses with energies from aboutseveral hundred pJ to about 1 nJ, and up to about 5 nJ. In someembodiments, the pulse energy at the output of the pre-amplifier 240 maybe above about 1 nJ, for example, in a range from about 1 nJ to about 20nJ, and in some embodiments up to about 100 nJ. In some embodiments alarge-mode amplifier may be used as a pre-amplifier so as to producemicrojoule seed pulses. For example, various amplifier options developedby the assignee of the present application (e.g.: amplifiers utilizing amultimode fiber, large core leakage channel fiber, photonic crystalfiber, and/or photonic bandgap fiber) that provide a high quality outputbeam are further described below and useable as either pre-amplifiers,power amplifiers, or generally as at least a portion of a multistageamplifier.

The pulse selector/modulator 244 may be configured to selectivelytransmit pulses to the power amplifier 248. The pulse selector/modulator244 may include an acoustic-optic modulator (AOM), an electro-opticmodulator (EOM), a high speed Mach-Zehnder device (MZ), and/or anelectro-absorption modulator (EAM). AOMs do not require high voltageelectronics, and commercially available digital driver electronicsprovide ease of use. Mach-Zehnder modulators (MZs) are integratedoptical devices having GHz bandwidths and low drive voltages, and inmany cases, require a polarized input beam. In some embodiments, therelatively small area of an integrated MZ device may limit useablepeak-power. In some embodiments, the pulse stretcher 236 reduces peakpower incident on the modulator 244, as described in the '327 patent. MZdevices have been used at 1.55 μm telecom wavelengths, and MZ devicesare now available at 1 μm wavelengths. The '327 patent discloses achirped pulse amplification system using MZ modulators. In certainembodiments, the pulse selector/modulator 244 may provide for about 20dB to about 30 dB of intensity control, and may be useable to at leastpartially control output intensity based on the transfer characteristicof the power amplifier 248 as a function of input.

In certain embodiments, the fiber power amplifier 248 comprises amultimode fiber amplifier configured to provide an output substantiallyin the fundamental mode. For example, the system may utilize a fiberpower amplifier as described in U.S. Pat. No. 5,818,630, issued toFermann, et al., entitled “Single-Mode Amplifiers and Compressors Basedon Multi-Mode Fibers,” assigned to the assignee of the presentapplication, and hereby incorporated by reference herein in itsentirety. Multimode fiber amplifiers provide production of peak powersand pulse energies that are higher than those achievable in single-mode(SM) fibers before the onset of undesirable nonlinearities and gainsaturation. In other embodiments, large-area amplifiers may be utilized,for example photonic bandgap or photonic crystal designs. High qualityoutput beams were demonstrated with leakage mode designs, for example,as described in U.S. patent application Ser. No. 11/134,856, entitled,“Single Mode Propagation in Fibers and Rods with Large LeakageChannels,” published as U.S. Patent Application Publication2006/0263024, assigned to the assignee of the present application, andhereby incorporated by reference herein in its entirety.

As described above, the compressor 252 is an all-fiber compressor insome embodiments. However, if peak power is too high, for example about100 kW or greater in some implementations, non-linear effects may limitperformance of an all-fiber compressor. A tradeoff may then existbetween the compactness of a fiber design and the flexibility associatedwith a bulk compressor. In some embodiments, both fiber and bulkcomponents may be used in the laser system 104.

The high repetition rate source 232 may produce pulses having an outputwavelength of about 1 μm. In some embodiments, the system 230 comprisesan optional frequency converter 256. For example, the frequencyconverter 256 may comprise a frequency doubler, a frequency tripler,and/or a frequency quadrupler producing respective visible (e.g., green)or ultraviolet output wavelengths (for 1 μm input wavelengths). In someembodiments the frequency converter 256 may comprise a parametricamplifier. Conversion efficiency is generally improved with higher peakintensity. Therefore, the frequency converter 256 advantageously may bepositioned to receive the output of the compressor 252. In one exampleembodiment, the frequency converter 256 was configured to providesecond, third, and fourth harmonic generation. Second harmonicgeneration was accomplished using a type I non-critically phase-matchedlithium triborate (LBO) crystal. The third harmonic was produced by sumfrequency mixing the fundamental and the second harmonic in a type IIcritically phase-matched LBO crystal. A type I LBO and type I betabarium borate (BBO) crystal can also be used in embodiments for 3rdharmonic generation, producing near UV output wavelengths. A type Icritically phase-matched beta barium borate (BBO) crystal generated thefourth harmonic by frequency doubling the second harmonic light. In thisexample embodiment, light having 50 μJ, 500 fs pulses at a fundamentalwavelength of 1040 nm was input to the frequency converter 256, whichprovided 53%, 25%, and 10% conversion efficiency to second, third, andfourth harmonic frequencies, respectively. At a laser repetition rate of100 kHz, this example embodiment produced an average power of about 5.00W at 1040 nm, and average converted powers of about 2.62 W at 520 nm,about 1.20 W at 346 nm, and about 504 mW at 260 nm. The converted pulseenergies were about 26 μJ at 520 nm, about 12 μJ at 346 nm, and about 5μJ at 260 nm. Further details of a laser system 104 that may be used forproviding frequency converted ultrashort pulses are described in “12 μJ,1.2 W Femtosecond Pulse Generation at 346 nm from a Frequency-tripled YbCubicon Fiber Amplifier,” by Shah, et al., 2005, CLEO 2005 Postdeadline,CPDB1, which is hereby incorporated by reference herein in its entirety.

The controller 114 may be used to coordinates the positioning of thescanning beam and the selection of laser pulses. In certain embodiments,when the high repetition rate source 232 is free-running, a portion ofthe beam is detected using a length of optical fiber coupled to a highspeed photodetector (not shown). The photodetector output provides asynchronization signal to the controller 114. The synchronization signaladvantageously may be a digital signal. The scanning system 106 mayinclude 2-D galvanometer mirrors 108 such as, for example, hurrySCAN® II14 scan heads available from SCANLAB America, Inc. (Naperville, Ill.).Advantages of using such scan heads include that they are lowinertia-devices and are provided with user-friendly commerciallyavailable controllers so that mirror position and/or velocity signalsare readily programmable. The scanning system 106 and the controller 114may also be used with any suitable combination of translation stages,rotation stages, and robotic arm (not shown) to position the targetsubstrate 112. In some embodiments the scanning mirrors 108 may beomitted and any other suitable system for relatively moving the laserbeam and the target substrate 112. Suitable focusing optics 110 such as,for example, an F-theta lens and/or a high resolution objective may beused to focus each laser pulse onto the surface of or in the targetmaterial. Some refractive optical elements may introduce spot placementand focusing errors, or other temporal or spatial distortions, resultingfrom material dispersion. In certain embodiments, commercially availableoptic elements designed for ultrashort laser pulse beams are used. Incertain embodiments, the controller 114 is configured to control spatialoverlap between adjacent focused laser pulses (or groups of laserpulses) during processing of the target material.

In certain embodiments, it may be desirable to operate the amplifier(s)substantially continuously to reduce the likelihood of damage and toprovide for maximum energy extraction from the amplifiers. Fiberamplifiers are well suited for amplifying high speed pulse trains.However, in some embodiments, increased risk of amplifier damage occursand undesirable amplified spontaneous emission (ASE) is generated duringextended periods where material processing does not occur (“idleperiods”). For example, in some amplifiers, the idle time period may bein a range from tens of microseconds to hundreds of milliseconds orgreater. In certain fiber amplifiers, an idle time of about 100 μs maybe sufficient for gain to increase to a sufficient level for free-lasingunder high gain (strong pumping) conditions. After about 25-40 μs ofidle time, if a seed pulse is injected, the built up gain in theamplifier may have sufficient gain to create a high energy pulse capableof inducing damage to the output fiber facet. Accordingly, in certainembodiments, stabilization and protection of the laser components isprovided by dynamic adjustment of the input pulse energy and/or controlof a pump diode current as described, for example, in U.S. patentapplication Ser. No. 10/813,173, to Nati, et al., entitled “Method AndApparatus For Controlling And Protecting Pulsed High Power FiberAmplifier Systems,” published as U.S. Patent Application Publication No.2005/0225846, assigned to the assignee of the present application, andhereby incorporated by reference herein in its entirety.

In various embodiments of the system 230, the controller 114 can beconfigured to operate the pulse selector/modulator 244 at a highrepetition rate (e.g., from about 50 MHz to about 100 MHz) during idleperiods. During idle periods, the amplifier 248 is generally operatingin a non-saturated regime. The power amplifier average output mayslightly increase at the fundamental wavelength. Modulation of the pulseenergy between an idle period and an “active” period (when the system230 is processing the target) may be sufficient to provide rapidshuttering of the beam (e.g., “off” and “on” functionality). In someimplementations, the laser fluence on the target substrate 112 duringsome “idle” periods may be above the ablation and/or surfacemodification thresholds, but the modulation in fluence between “idle”and “active” periods may be sufficient for process control. In someembodiments, an optional shutter 260 may be used to control the energyincident on the target substrate 112. The optional shutter 260 maycomprise an acousto-optic device, an opto-mechanical shutter, and/or anelectro-optic shutter.

Certain embodiments of the system 230 include a frequency converter 256that may provide, for example, frequency doubling and/or tripling. Incertain such embodiments, the pulse energy and/or the peak power may berelatively low at the output of the frequency converter 256. In suchcases, output of the converter 256 may be a relatively low energy pulsewith most energy content at the fundamental wavelength and, at focus onthe target; the energy may be below the ablation and/or surfacemodification thresholds of the target material. In some systemembodiments, modulator adjustment of about 20 dB to about 30 dB mayprovide control of intensity over a wide operating range so as to avoidaltering target material properties.

In certain embodiments, techniques may be used to attenuate unwantedbeam energy. For example, unwanted energy may be removed with a spectralfilter (not shown). In some implementations, polarization filtering maybe possible, because of the difference in polarization state betweenfundamental and harmonic frequencies for Type I phase matching. Thepulse selector/modulator 244 also may be controlled to limit the energyto the amplifier 248. Focusing optics in the scanning system 106 (orother focusing optics if a scanner is not used) may be optimized for themachining wavelength (which may be a frequency converted wavelength ifthe optional frequency converter 256 is used). In some implementations,the focusing optics may be configured so that the spot size of thefundamental wavelength is increased so that the energy density at thesurface of the target substrate 112 is reduced.

During active processing periods, the controller 114 may be used toprovide signals to the pulse selector/modulator 244 to “down count” orotherwise select pulses. In some embodiments, processing repetitionrates may be from about 100 KHz to about 10 MHz. During activeprocessing, it may be advantageous for the laser to operate insaturation, or approximately so, in order to extract the maximum energyfrom the fiber amplifier.

FIG. 3 schematically illustrates an embodiment of a system 300 capableof use for machining a workpiece (or target substrate) 112 via ultrafastpulse trains. This system 300 may be generally similar to theembodiments depicted in FIGS. 1F and 2A, 2B. The system 300 may furthercomprise a robotic arm system 304 coupled to the target substrate 112and configured to manipulate the target position (and/or orientation)relative to the scanning beam. The robotic arm system 304 may be asingle-axis or a multi-axis system. In some embodiments, the scanningsystem 106 comprises a scan head that is moved with respect to thetarget substrate 112. A possible advantage of embodiments providingrelative movement between the scan beam and the target substrate 112 isthat the system may enable processing of non-flat surfaces.

In some embodiments of the systems 100, 200, 230, and 300 schematicallyshown in FIGS. 1-F, 2A, 2B and 3, respectively, the laser spot size isprimarily determined by the F-theta lens in the scanning system 106. Insome implementations, in order to have reasonable processing area forimages, spot sizes larger than about 10 μm are used. Certain embodimentsof the laser system 104 are capable of machining much smaller spot sizes(e.g., ≦1 μm). For such small focal dimensions, significantly lowerpulse energy is used in some embodiments. In order to achievesufficiently high resolution over a sufficiently large working area, thetarget and the beam may be moved with respect to each other. Forexample, the target may be moved relative to a substantially stationarylaser beam (or vice-versa).

In certain embodiments of the systems 100, 200, 230, and 300, a variabletelescope can be positioned along an optical path between the lasersystem 104 and the scanning system 106. In certain such embodiments, theF-theta lens may be omitted from the scanning system 106. The variabletelescope may be used to dynamically vary the focal length of the systemand may provide continuous variation of the focal spot size on thetarget substrate 112. A commercially available variable telescope systemmay include, for example, the varioSCAN dynamic focusing unit availablefrom SCANLAB America, Inc. (Naperville, Ill.). Such a system, withdynamic focusing, provides capability for 3D adjustment of the beamfocal position, and useful capability for following or compensatingvariations in the target surface locations, as might be caused bysubstrate warpage or other deviations from flatness.

FIG. 4A schematically illustrates an embodiment of a system 400 capableof use for processing semiconductor substrates with ultrafast pulsetrains. This embodiment comprises a laser system 104 and a translationstage 408 configured to move the target substrate 112 relative to thelaser beam. In certain embodiments, the translation stage 408 remains insubstantially constant motion with relatively high translation speeds inorder to enable sufficiently high processing speeds. In someembodiments, the translation stage 408 may include an X-Y or an X-Y-Ztranslation stage. For example, the translation stage 408 may include aNano-Translation (ANTTM) stage available from Aerotech, Inc.(Pittsburgh, Pa.). Many techniques for relatively controllingpositioning of a pulsed laser beam and a target substrate are known suchas, for example, as described in U.S. Pat. No. 6,172,325 to Baird, etal., entitled “Laser Processing Power Output Stabilization Apparatus andMethod Employing Processing Position Feedback.” In some embodiments, thecontroller 114 may execute control instructions for coordinating thescanning system 106 and the translation stage 408 such as, for example,the Nmark™ control software available from Aerotech, Inc. (Pittsburgh,Pa.).

In some embodiments of the system 400 schematically shown in FIG. 4A, amodulator 402 may be used to provide substantially instantaneous lasermodulation for improved control of the laser-material interaction. Themodulator 402 may be generally similar to the modulator 202 describedwith reference to FIG. 2, or the modulator 402 may be an externalmodulator as schematically depicted in FIG. 4A. In certain embodiments,the controller 114 provides linked control of the modulator 402 and thetranslation stage 408.

In certain embodiments, the systems described herein (e.g., the systems100, 200, 230, 300, and 400) may process a target substrate usingmultiple passes of a laser beam relative to the target substrate. Forexample, ten or more passes may be used in various embodiments, andperhaps hundreds for formation of very high aspect ratio features. Thefluence (and/or other system parameters) may be adjusted to control thematerial removal during a given pass.

In various embodiment the system the system may utilize informationregarding the state of the laser system of target and, based on feedbacksignals, control laser parameters as described in, for example, U.S.patent application Ser. No. 10/813,269, filed March 31, 204, entitled“Femtosecond laser processing system with process parameters, controlsand feedback,” (hereinafter, the '269 application) assigned to theassignee of the present application, and which is hereby incorporated byreference in its entirety.

In some embodiments, a system may be provided wherein each laser pulsemay have individualized characteristics. At least one of the laserpulses may be an ultrashort pulse. The system may comprise a laser meansfor generating a pulse or high repetition rate bursts of pulses asprovided in one or more of the embodiments 100, 200, 230, 300, 400.Additionally a control means that controls the laser means and a beammanipulation means for monitoring the pulse width, wavelength,repetition rate, polarization, and/or temporal delay characteristics ofthe pulses comprising the pulse bursts may be included. In someembodiments, the system may generate feedback data based on the measuredpulse width, wavelength, repetition rate, polarization and/or temporaldelay characteristics for the control means. In one embodiment, thelaser means may comprise a fiber amplifier that uses stretcher gratingsand compressor gratings. The beam manipulation means can comprise avariety of devices including, e.g., an optical gating device thatmeasures the pulse duration of the laser pulses, a power meter thatmeasures the power of the laser pulses output from the laser means,and/or a photodiode that measures a repetition rate of the laser pulses.In some embodiments where a frequency converter is utilized, for examplea doubler or tripler, a beam manipulation means optically converts thefundamental frequency of a percentage of the generated laser pulses toone or more other optical frequencies, and includes at least one opticalmember that converts a portion of the fundamental of the laser pulsesinto at least one higher order harmonic signal. The optical member maycomprise a non-linear crystal device with a controller that controls thecrystal's orientation. In certain embodiments, the means for convertingan optical frequency advantageously includes a spectrometer thatmeasures one or more predetermined parameters of pulses output from thenon-linear crystal device and generates feedback for the control means.Another embodiment of the beam manipulation means comprises telescopicoptical devices to control the size, shape, divergence, and/orpolarization of the laser pulses input, and steering optics to controlan impingement location of the laser pulses on a target substrate. Thesystem may further comprise a beam profiler that monitorscharacteristics of laser pulses and generates feedback for the controlmeans. The above-described system has several uses including, but notlimited to, modifying the refractive index of a target substrate;surface marking, sub-surface marking, and/or surface texturing of atarget substrate; fabricating holes, channels, trenches, grooves, vias,and/or other features in a target substrate; and depositing and/orremoving thin layers of material on a target substrate.

As shown in the embodiment of a laser processing system illustrated inFIG. 5, the control means 5300 is coupled to the laser means 5100. Thelaser system may be generally similar to an embodiment of the lasersystem schematically illustrated in FIG. 5 of the '269 application. Thecontrol means 5300 monitors several output laser parameters, such as,for example, the average output power, the pulse train (repetition rateand/or burst mode structure), pulse duration (and/or temporal phase,e.g., FROG, frequency resolved optical gating), and/or spatial phase(wavefront sensor). The monitored parameters are linked to the controlmeans 5300 in order to vary laser performance (pulse energy, repetitionrate and pulse duration) through feedback loops. Furthermore, thefeedback loops may be linked to compressor alignment (e.g., gratingseparation) in order to pre-chirp the laser pulse, thereby compensatingfor the optical dispersion caused by the components in subsequent lasersystem modules. The control means 5300 may comprise, for example, adesktop computer, a laptop computer, a tablet computer, a handheldcomputer, a workstation computer or any other general-purpose and/orspecial-purpose computing or communicating device. The control means5300 may execute any of the well-known MAC-OS, WINDOWS, UNIX, LINUX, orother appropriate operating systems on a computer (not shown). Thecontrol means 5300 may be networked to other computing means by physicallinks and/or wireless links. The control means 5300 may comprise aninput device, an output device, random access memory (RAM), and/orread-only memory (ROM), a CD-ROM, a DVD device, a hard drive, and/orother magnetic or optical storage media, or other appropriate storageand retrieval devices. The control means 5300 may also comprise aprocessor having a system clock or other suitable timing device orsoftware. The input device might comprise a keyboard, mouse, a touchscreen, pressure-sensitive pad or other suitable input device, and theoutput device can comprise a video display, a printer, a disk drive orother suitable output device.

In some embodiments, additional tools may be included to monitor thestatus of the target substrate, and/or to confirm/control the focalposition relative to the surface of the target substrate. For example,an illumination and optical microscopic viewing system (not shown) couldbe used to locate alignment markers, confirm/deny laser damage, andmeasure laser affected feature volume and/or morphology. Additional datacould be obtained by including spectroscopic diagnostics such as laserinduced breakdown spectroscopy (LIBS) and/or laser-induced fluorescence.A range-finding tool that precisely determines the distance from thetarget surface to the focal point could also be employed. In someapplications, determining the distance may be advantageous since oneapplication may include micron-level material processing. Use of acamera system that images the surface of the target substrate could beused as well. At these dimensions, small error/uncertainty may reducethe user's ability to precisely control the laser/material interaction.This may be complicated since several such applications potentiallyinvolve sub-surface processing of materials with non-planar surfaces.Signals from the viewing/spectroscopic tools could feedback to othersystem components (e.g., the control means, the means for convertingoptical frequencies, etc.) to precisely influence the extent and natureof the laser/material interaction. Furthermore, the signal from therange finding tool and/or the viewing/spectroscopic tools can be fedback to the control workpiece positions. The scanning mechanism steeringoptics, which may include a galvanometer based mirror scanner andpossibly one or more additional precision positioners, and control means5300 provide that the beam is accurately delivered to the targetsubstrate.

Accordingly, in certain embodiments of the systems described herein,laser controls and diagnostics allow for active control of processingparameters in order to ensure that the laser intensity remains within adesired (and/or an optimal range), thereby assuring consistent featuresize, material removal rate, and thermal effect. In addition, theability to control the size, shape, divergence, and/or polarization ofthe beam makes it possible to further improve (and/or optimize) theshape and/or edge quality of machined features (such as, e.g., groovesand/or trenches). For example, it has been demonstrated that the use ofa highly elliptical beam with its major axis parallel to the directionof translation is capable of producing trenches with higher aspect ratioand better surface quality than is possible using a round focal beam(see, e.g., Barsch, Korber, Ostendorf, and Tonshoff, “Ablation andCutting of Planar Silicon Devices using Femtosecond Laser Pulses,” Appl.Physics A 77, pp. 237-244, (2003) and Ostendorf, Kulik, and Barsch,“Processing Thin Silicon with Ultrashort-pulsed Lasers Creating anAlternative to Conventional Sawing Techniques,” Proceedings of theICALEO, Jacksonville, USA, October 2003). Adjusting the laserpolarization relative to the direction of scanning has also been shownto affect the surface and edge quality of femtosecond machined grooves.The ability to actively monitor and independently control laser and beamparameters, as enabled by various embodiments of the laser systemsdescribed herein, is beneficial for achieving reproducible micron-levelprecision in the fabrication of features including, for example, surfacegrooves and/or trenches.

Further details of a system having feedback and controls are describedin the '269 application, such as, for example, FIGS. 7-13 and thecorresponding text of the '269 application.

In some embodiments, processing may be carried out with a train ofpicoseconds pulses having total energies sufficient for materialremoval. For example, pulse widths may be in a range of about 10 ps toabout 500 ps. In some embodiments, a pulse compressor may not beutilized. In such embodiments, pulses from one or more laser sources maybe amplified to produce the processing pulses. Such a configuration maybe generally similar to embodiments of the systems 100, 200, 230, 300,and 400, but with omission of a pulse compressor.

Embodiments are applicable for many micromachining applications, andwell matched to applications in microelectronics including, for example,wafer cutting, dicing, scribing, and similar applications. In someapplications, suitable modifications of elements shown in the systemembodiments 100, 200, 230, 300, and 400 may be made using methods anddevices adapted for such applications. For example, in one embodiment, asubstrate positioning mechanism may include the X-Y-Z stage 408, andadditional rotation mechanism(s) to provide 6-axis capability and/or tomaintain flatness and coplanarity of the target substrate (e.g., awafer). For example, the wafer may be mounted to a special holder (e.g.,a wafer chuck) by another material (e.g., tape) for a cutting operation.

Certain embodiments of the systems 100, 200, 230, 300, and 400 includevarious combinations of laser and amplifier(s). Although fiber-basedtechnology is preferred in some embodiments, various embodiments mayutilize waveguide lasers and/or amplifiers, regenerative amplifiers, andso forth. In some embodiments, the technologies may be used incombination with fiber amplifiers, lasers, and/or a length of un-dopedtransmission fiber. For example, in one embodiment, a passivelyQ-switched microchip laser may produce several microjoules of pulseenergy at repetition rates somewhat below one MHz, for example up toabout 100-500 kHz, and somewhat larger. Pulse widths may be in the rangeof about 1 ps to about 100 ps. In some embodiments a microchip laser mayseed a fiber amplifier, for example as disclosed in theabove-incorporated U.S. patent application Ser. No. 10/437,057 toHarter.

In some embodiments, pulse widths of less than a few nanoseconds may beutilized, for example sub-nanosecond pulses or pulses having a width ofabout 500 ps or less. Suitable modifications of the embodiments shown in100, 200, 230, 300, and 400 include diode based or microchip laser seedsources, elimination of at least one of a pulse stretcher and pulsecompressor, reduced number of amplifier stages, elimination of amplifierstages, and the like.

For example, the above-incorporated U.S. patent application Ser. No.10/437,057 discloses various embodiments utilizing seed and microchiplaser sources which are amplified and compressed with various fiber andnon-fiber elements to produce ultrashort pulse widths. In one embodimentseed pulses of a few nanoseconds are generated using a semiconductorlaser diode, portions thereof are selected using a GHz electro-opticmodulation, and then further processed so as to obtain amplified andcompressed pulses. Typical repetition rates are less than about 10 MHz.The disclosed arrangements provide elements and sub-systems useable by aperson skilled in the art to construct relatively high repetition rate(e.g., 500 kHz-10 MHz) short pulses (sub-picosecond to about a fewnanoseconds) so as to create geometric features within a predeterminedtolerance, and with reduced accumulation of redeposited material on orvery near to a processing location when operated at the higherrepetition rates.

Numerous variations are possible. For example, in some embodiments aQ-switched microchip laser may provide pulses having a width of a fewpicoseconds to several tens of picoseconds, but at a rate of tens of kHzup to about 100 kHz. In some embodiments, the operating repetition ratemay be increased substantially, for example to 500 kHz or a few MHz,with a tolerable increase in the pulse width to a sub-nanosecond width.An embodiment may optionally include a fiber amplifier. By way ofexample, R. Fluck, B. Braun, E. Gini, H. Melchior, and U. Keller, in“Passively Q-switched 1.34 μm Nd: YVO₄ Microchip Laser withSemiconductor Saturable Absorber Mirrors, Optics letters, Vol. 22, No.13, disclose an early 1.3 μm passively Q-switched laser with performancesimilar to a 1 μm version. Variations in pump power, crystal length, anddesign of a SESAM (semiconductor saturable absorber mirror) resulted inpulse widths of 230 ps to 12 ns and repetition rates of 30 kHz to 4 MHz.

Some embodiments may include modifications of an early model WO-pulsardeveloped by IMRA America, Inc, the owner of the present application.The system utilized a semiconductor seed laser and at least one fiberoptic amplifier. Numerous other configurations are possible.

Various embodiments may provide pulse widths in the range of about a fewhundred picoseconds to several hundred picoseconds. The laser system maycomprise an FCPA system. In some embodiments that utilize picosecond orlonger pulses any pulse compressor may be eliminated. Alternatively, asystem may be configured to produce amplified picosecond pulses withoutchirped pulse amplification, e.g., preferably with a fiber amplifiersystem.

Referring back to FIG. 4A, the laser system 104 may include amode-locked fiber oscillator or other seed source, and a fiber amplifiersystem having a fiber power amplifier. In various embodiments the fiberpower amplifier comprises a multimode fiber amplifier configured toprovide an output substantially in the fundamental mode. For example,the system may utilize a fiber power amplifier as described in U.S. Pat.No. 5,818,630, issued to Fermann, et al., entitled “Single-ModeAmplifiers and Compressors Based on Multi-Mode Fibers,” assigned to theassignee of the present application, and hereby incorporated byreference herein in its entirety. Multimode fiber amplifiers provideproduction of peak powers and pulse energies that are higher than thoseachievable in single-mode (SM) fibers before the onset of undesirablenonlinearities and gain saturation. In other embodiments, large-areaamplifiers may be utilized, for example photonic bandgap or photoniccrystal fiber designs. High quality output beams were demonstrated withleakage mode designs, for example, as described in U.S. patentapplication Ser. No. 11/134,856, entitled, “Single Mode Propagation inFibers and Rods with Large Leakage Channels,” published as U.S. PatentApplication Publication 2006/0263024, assigned to the assignee of thepresent application, and hereby incorporated by reference herein in itsentirety.

In at least one embodiment the amplifier may be included in laser system104. FIG. 4B schematically illustrates an example embodiment 470 of alarge mode area fiber comprising a core doped with rare earth ions thatcan be used in a fiber amplifier, or in a laser pumped by a multimodepump source. For example, the embodiment 470 can be included in thelaser system 104 schematically shown in FIG. 4A. Input beam 456 may begenerated with a mode-locked oscillator, semiconductor diode, diode andelectro-optic modulator, and/or other suitable source. Prior toamplification with the large-mode amplifier, a preamplifier may boostthe pulse energy levels. The laser system 104 may also include pulseselectors, polarization controllers, and/or beam shaping optics tocondition pulses prior to and/or after amplification. In the embodiment470 illustrated in FIG. 4B, the fiber 450 has straight input and outputends 451, 452, respectively, and a coiled section therebetween. Amultimode pump 455 is used to pump the amplifier or laser using acoupling lens 454. Input beam 456 is launched into the fiber 450 througha lens 453. Output 457 is separated by dichroic mirror 458. In otherembodiments, the fiber 450 and/or other components may be configureddifferently than schematically shown in FIG. 4B. Also, variouscomponents can be removed, added, and/or arranged differently than shownin the example embodiment 470 illustrated in FIG. 4B.

Various laser or amplifier embodiments may be utilized in an all-fiberdesign for generation of high peak power pulses in the femtosecond,picosecond, and/or nanosecond regimes with reduced or negligiblenon-linear effects. The laser or amplifiers may also be utilized in FCPAsystems to further increase available pulse energy. In one exampleembodiment a core diameter of about 70 μm-100 μm or larger may be usedto produce nanosecond pulses having energy of a few mJ to about 10 mJ.In another embodiment, ultrashort pulses with example pulse widths ofabout 1-10 ps may be produced with output energies in the range of about10 μJ to a few hundred μJ. Pulse repetition rates greater than about 100KHz, and up to at least a few MHz may be utilized in variousembodiments. Repetition rates in the range of 100 MHz to 1 GHz are alsopossible, depending on the average power rating and system requirements.By way of example, and as disclosed in PCT Application No.PCT/US2008/074668 entitled “Glass Large-Core Optical Fibers”, which isowned by the assignee of the present application and which is herebyincorporated by reference herein in its entirety, input pulses at 25 KHzrepetition rate, 5 μJ pulse energy, and 600 ps pulse width emitted froma microchip laser were amplified to about 400 μJ in a large-core leakagechannel fiber, producing nearly diffraction limited output beams, andwithout distortion caused by non-linear effects. In the experiment nopulse stretching or compression was utilized. Higher pulse energies andrepetition rates are achievable. Therefore, many possibilities exist forfiber-based amplified laser configurations.

Experimental Results—Silicon Trench Digging and Wafer Dicing Examples

The example experimental results that follow demonstrate embodiments ofsystems and methods for “trench digging” and “wafer dicing.” In theseexample experiments, silicon substrates were generally processed usingultrashort pulses having sub-picosecond pulse widths. Some results wereobtained with pulses having a pulse width up to about 200 ps.

Experimental System

As schematically illustrated in FIG. 6A, the experimental arrangementincluded a laser system 104 providing more than 10 μJ of available pulseenergy, pulse widths in a range from about 500 fs to about 500 ps, and arepetition rate exceeding 100 kHz. The laser parameters of theexperimental system were varied, although not necessarily all theparameters were independent of each other. For example, pulse energyinfluences the minimum achievable pulse width. In some experiments, atypical focused spot size at the substrate surface was about 15 μm,which at an energy of 10 μJ provides an average fluence of about 5-6J/cm² and a power density of nearly 10¹³ W/cm². At a repetition rate of500 kHz, the average power produced by the system is about 5 W in someexperiments.

A photograph of the system is shown in FIG. 6B. The experimental setupincludes a fume extractor 614, which is operable to remove particulatefumes generated during machining. The fume extractor 614 was operated atan extraction rate of approximately 350 cubic feet per minute. Theexperimental setup included an XYZ motion system 610.

Measurement Tools, Methods, and Specifications

FIG. 7 schematically illustrates one example technique for quantifyingprocessing quality so as to obtain an approximation of an ablated volumeand a redeposited volume proximate to a processing location. For someprocessing applications, a rough measurement of the trench depthrelative to the recast peak or average height may be sufficient toquantify the influence of laser parameters on machining quality.However, for other processing applications, it may be more accurate tocharacterize a quantity of material removed versus a quantity ofmaterial recast.

As an example, FIG. 7 schematically illustrates one possible method forquantifying laser-based material processing. From cross-sectional imagesof the trench obtained, for example, with a scanning electron microscope(SEM), the ablated area is approximated by fitting a triangle to theshape of a vertical bisector of the trench (hatched triangles in FIG.7). A triangular approximation is also used to approximate the amount ofmaterial recast (cross-hatched triangles in FIG. 7). The trench (orother feature) quality is therefore quantified by the ratio of thetriangular ablated area to the triangular recast area. In the followingexample results, a JEOL JSM 6060 SEM, available from JEOL USA, Inc.(Peabody, Mass.) was used. Additional measurements are obtainable usingautomated or semi-automated tools, for example a fully automated SEMsystem, white light interferometer, surface profilometer(s), and/oratomic force microscopes.

Some of the measurements, particularly those with longer pulse widths of200 ps, were made by approximating the trench and recast with a polygonenclosing the area and estimating the area from the area of the polygon.In various embodiments, the polygon may have 3, 4, 5, 6, 7, 8, or moresides. In other embodiments, different shapes may be used to estimatethe trench and/or recast areas. For example, a cross-sectional profileof a feature may be approximated by a spline, a best-fit curve, etc. Inyet other embodiments, trench and/or recast areas may be estimated bysumming areas of a plurality of geometric shapes (e.g., rectangles,trapezoids, etc.) that “fit” in the features (e.g., generally similar toapproximating the area under a curve using the trapezoidal rule orSimpson's rule). A wide variety of numerical techniques may be used toestimate areas.

FIGS. 7A-7F show example SEM photographs of cross-sections obtained fromsilicon samples. The photographs demonstrate the trends observed in theexperiments as will be described below. FIGS. 7A-7F include values oflaser parameters, which are approximate, but known with sufficientaccuracy to support any conclusions set forth below. FIGS. 7A-7F includescale bars to indicate the sizes of various features shown in the SEMphotographs.

Reference is made below and in the corresponding figures to the numberof machining (or processing) passes, N at various scan speeds. Invarious embodiments, the scanner arrangement provides for a scan speed(measured for example in m/s), but, in certain embodiments, a retraceperiod may occur at a fixed rate (10 m/s in the example experiments).Because the laser source was active during the retrace period in theexample experiments described below, the laser exposure is somewhatgreater than the listed pulse energies, and the additional exposure canbe determined from the ratio of the 10 m/s retrace speed to the listedscan speed.

Example Effects of Varying Pulse Energy

FIGS. 7A and 7B illustrate some example experimental effects of varyinglaser energy. In these examples, a 200 kHz laser repetition rate, a 15μm focal spot diameter, and about 33% spatial overlap were fixed duringmachining. FIG. 7A shows results for pulse energies of about 10 μJ, andFIG. 7B shows results for pulse energies of about 20 μJ. The imagepanels in FIGS. 7A and 7B show results for 100 passes, 200 passes, and400 passes. Increasing pulse energy from 10 μJ to 20 μJ provides forgreater depth penetration with an increasing number of machining passes.The larger pulse energy also produces a greater amount of redepositedmaterial surrounding the ablated feature. For example, after 400 passesusing 20 μJ pulse energy, which generally corresponds to the largestmachining depth in this example, the machining produced about 15 μmpeak-to-peak of redeposited material. In contrast, with 10 μJ pulses,only about 10 μm peak-to-peak of redeposited material was measured.Therefore, greater ablated volume results in greater redeposited volume.

Example Effects of Scan Speed and Efficiency Versus Unwanted RedepositedMaterial

FIG. 7C includes SEM photographs showing effects of scan speed,efficiency, and accumulation of redeposited material. In this example, a500 kHz laser repetition rate, a 15 μm 1/e² focal spot diameter, a 10 μJpulse energy, and 100 passes were fixed parameters. The scan speed wasvaried in a range from about 0.5 m/s to about 8 m/s. As the scan speedincreases in this range, the spatial overlap of incident pulses and thetotal incident energy decreases, and both the ablated volume and theredeposited volume are reduced. Decreasing the spatial overlap reducesthe heat load per unit area and reduces the interaction of subsequentlaser pulses with “excited” material. As the scan speed increases inthis range, the ratio of ablated depth to recast height increases, whichresults in cleaner processing. This trend is not unusual, and iscommonly observed during high speed/multi-pass cutting and helicaldrilling applications with nanosecond solid-state laser systems.

Example Effects of Laser Repetition Rate

FIGS. 7D-7F illustrate example effects of laser repetition rate on themachining quality. In this example, the fixed focal spot diameter ofabout 15 μm at 1/e², the pulse energy of 20 μJ, and 200 machining passeswere fixed parameters. In each of FIGS. 7D-7F, image panels show resultsin which the scan speed was varied from about 0.5 m/s to about 10 m/sThe experimental results shown in FIGS. 7D-7F were performed at a laserrepetition rate of about 200 kHz, 350 kHz, and 500 kHz, respectively.FIGS. 7D-7F include scale bars to indicate the size of ablated andredeposited cross sections. In this example pulse energy of about 20 μJwas used and both the scan speed and repetition rate varied.

The depth and area of ablated features and the height and areas ofrecast material were measured using the technique described withreference to FIG. 7, and the results of the experiments are summarizedin the tables below. In these example tables, a measure of processingquality is the ratio of the ablated area of a feature to the area ofmaterial redeposited adjacent to the ablated feature. Processingefficiency may be characterized in terms of processing speed (e.g.,ablated area per second) relative to the average power (generallyassuming the same pulse energy). The experimental data shown in FIGS.7D-7F and summarized in the example tables were obtained by varyingrepetition rate and scan-speed with other parameters fixed. This datamay be used to determine an example relation between quality andprocessing efficiency for the experiments summarized in the tables.

Tables Corresponding to Results Shown in FIGS. 7D-7F 350 kHz, 20uJ 200kHz, 20uJ ablation speed ablation recast speed depth recast (m/s) depth(um) height (um) ratio (m/s) (um) height (um) ratio   0.5 85 12 7.0833330.5 100 12 8.333333 1 72 12 6 1 90 8 11.25 2 40 10 4 2 75 8 9.375 5 304.5 6.666667 5 45 5 9   7.5 15 3.5 4.285714 7.5 40 4.5 8.888889 10  13 34.333333 10 30 4 7.5 200 kHz, 20uJ 350 kHz, 20uJ ablated recast ablatedrecast speed area area speed area area (m/s) (um{circumflex over ( )}2)(um{circumflex over ( )}2) ratio (m/s) (um{circumflex over ( )}2)(um{circumflex over ( )}2) ratio   0.5 744 300 2.48 0.5 875 270 3.2407411 630 270 2.333333 1 788 140 5.628571 2 350 225 1.555556 2 656 1404.685714 5 278 101 2.752475 5 394 88 4.477273   7.5 135 70 1.928571 7.5350 79 4.43038 10  114 60 1.9 10 263 60 4.383333 500 kHz, 20uJ ablationrecast speed depth height (m/s) (um) (um) ratio   0.5 120 12 10 1 95 127.916667 2 75 9 8.333333 5 55 5 11   7.5 50 4.5 11.11111 10  43 4 10.75500 kHz, 20uJ ablated recast speed area area (m/s) (um{circumflex over( )}2) (um{circumflex over ( )}2) ratio   0.5 1050 300 3.5 1 831 1804.616667 2 656 135 4.859259 5 412 63 6.539683   7.5 438 50 8.76 10  32346 7.021739

FIG. 8 is a plot showing the ratio of the ablated cross sectional areato redeposited cross sectional area relative to scan speed at thedifferent laser repetition rates for the experimental results shown inFIGS. 7D-7F. This plot demonstrates that the quality of the ablatedfeatures improves with increasing repetition rate for scan speedsgreater than about 2 m/s.

Examples of processing efficiency are shown in FIG. 9. A weightedablated cross sectional area is plotted versus scan speed for theresults shown in FIGS. 7D-7F. In order to account for differences inaverage power and spatial overlap, the ablated areas are multiplied by aweighting factor which is proportional to the differences in laserrepetition rate. The weighting factor is 1 for 200 kHz, 0.57 for 350kHz, and 0.4 for 500 kHz. The plot in FIG. 9 demonstrates that theefficiency of ablation is independent of repetition rate for scan speedsgreater than 2 m/s. Therefore, the improvement in processing quality (athigher repetition rates) is not significantly compromised at the expenseof processing efficiency.

Increasing the laser repetition rate significantly affects the natureand amount of redeposited material. Referring back to FIGS. 1A-1E, theexperimental results demonstrate a desirable increase in the ratio ofmaterial removed from the target region 1001-c to the redepositedmaterial 1005-b. In contrast to the predictable trends shown in FIGS.7A-7C, the observed influence of high repetition rates (see, e.g., FIGS.7D-7F) was unforeseen and unexpected.

Although it is not necessary to the practice of embodiments of thedisclosed systems and methods to understand the operative mechanism forthese unexpected results and without subscribing to any particulartheory, the demonstrated improvement with increasing repetition rate maybe a result of the interaction between the ablation plume and subsequentlaser pulses as the inter-pulse separation decreases from about 10 μs toabout 1 μs. This may suggest, in some experiments, that the redepositedmaterial may comprise fine particles. Further, the results suggest apreviously unexploited laser-material interaction regime may exist,influenced by the repetition rate.

Examples of Cleaning and Post-Processing

FIGS. 10A-1 and 10A-2 show example SEM cross-sections, wherein aquantity of re-deposited material is sufficiently low such thatconventional ultrasonic cleaning is effective for further debrisremoval. These experimental results may be applicable to, for example,thin-wafer dicing and similar applications. The fixed laser parameterswere a 500 kHz repetition rate, a 10 μJ pulse energy, and 100 machiningpasses. The scanning speed was varied from about 0.5 m/s to about 8 m/sfor the experimental results shown in FIGS. 10A-1 and 10A-2. The SEMimages in the left panels of FIGS. 10A-1 and 10A-2 are before ultrasoniccleaning, and the SEM images in the right panels of FIGS. 10A-1 and10A-2 are after ultrasonic cleaning. The SEM photographs in FIGS. 10A-1and 10A-2 generally show a significant decrease in the volume of theredeposited material after cleaning.

FIG. 10B is a plot of a ratio of ablated depth to recast heightcorresponding to the data shown in FIGS. 10A-1 and 10A-2. The plot inFIG. 10B shows a nominal two-fold reduction in recast height afterultrasonic cleaning. In some cases, the height of redeposited materialremaining after cleaning was at or near a practical measurement limit ofthe method described with reference to FIG. 7, for example, about 1 μm,at 1000× magnification. The experimental results of FIGS. 10A-1, 10A-2,and 10B are summarized in the tables below.

Tables Corresponding to Results Shown in FIGS. 10A-1, 10A-2, and 10B 500kHz, 10 uJ, before cleaning abla- 500 kHz, 10 uJ, after cleaning tionrecast ablation recast speed depth height speed depth height (m/s) (um)(um) ratio (m/s) (um) (um) ratio 0.5 80 7 11.42857 0.5 80 3 26.66667 160 5 12 1 60 2 30 2 60 4 15 2 60 1 60 4 40 2 20 4 40 1 40 8 25 2 12.5 825 1 25

Examples of “Double-Pulse” Experiments

“Double pulse” experiments were also performed. In these experiments,single pulses and pairs of pulses were produced at a repetition rate ofabout 1 Mhz. The temporal spacing between pulses of each pair was about20 ns, corresponding to an instantaneous burst repetition rate of about50 MHz. FIGS. 11A-11C show example SEM cross-sections comparing resultsof single and double pulse processing. FIGS. 11D-11E are plots showingthe ratio of ablated depth to recast height, corresponding to the SEMimages of FIGS. 11A-11C. FIGS. 11A and 11B show results for experimentswith “single pulses” produced a repetition rate of about 1 MHz. Thepulse energy was about 5 μJ in FIG. 11A and about 10 μJ in FIG. 10B.FIG. 11C shows results for experiments with “double pulses” produced ata repetition rate of about 1 MHz and an instantaneous burst repetitionrate of about 50 MHz. The scan speed was varied in a range from about0.5 m/s to about 10 m/s in the experiments shown in FIGS. 11A-11C. Inall the experiments, 200 machining passes were used. A comparison of theimages in FIGS. 11A, 11B to the images in FIG. 11C indicates decreasedperformance of the double pulses compared to single pulses. The plots inFIGS. 11D and 11E also demonstrate decreased performance of the doublepulse experiments. These results may suggest that very highinstantaneous repetition rates, for example greater than about 10 MHzmay limit machining performance in silicon for these laser systemparameters (particularly at moderate to high scan speeds). The resultsare further summarized in the tables below.

Tables Corresponding to Results Shown in FIGS. 11A-11E 1 MHz, 5 uJ,single pulse 1 MHz, 5 uJ, double pulse ablation recast ablation recastspeed depth height speed depth height (m/s) (um) (um) ratio (m/s) (um)(um) ratio 0.5 30 7 4.285714 0.5 70 15 4.666667 1   30 7 4.285714 1 5515 3.666667 2   33 6 5.5 2 40 11 3.636364 5   33 4 8.25 5 35 7 5 7.5 334 8.25 7.5 30 9 3.333333 10   30 4 7.5 10 30 6 5 1 MHz, 5 uJ, singlepulse 1 MHz, 5 uJ, double pulse ablated recast ablated recast speed areaarea speed area area (m/s) (um{circumflex over ( )}2) (um{circumflexover ( )}2) ratio (m/s) (um{circumflex over ( )}2) (um{circumflex over( )}2) ratio 0.5 210 225 0.933333 0.5 525 562 0.934164 1   210 140 1.5 1412 375 1.098667 2   231 51 4.529412 2 280 220 1.272727 5   231 21 11 5245 122 2.008197 7.5 231 34 6.794118 7.5 210 122 1.721311 10   210 287.5 10 187 105 1.780952 1 MHz, 10 uJ, single pulse ablation recast speeddepth height (m/s) (um) (um) ratio 0.5 70 15 4.666667 1   55 11 5 2   509 5.555556 5   45 5.5 8.181818 7.5 45 5 9 10   40 5 8 1 MHz, 10 uJ,single pulse ablated recast speed area area (m/s) (um{circumflex over( )}2) (um{circumflex over ( )}2) ratio 0.5 525 562 0.934164 1   412 2751.498182 2   375 135 2.777778 5   337 69 4.884058 7.5 337 62 5.43548410   560 50 11.2

Example Experimental Results for Wafer Dicing and Ultrasonic Cleaning

FIGS. 12A-12B illustrates example SEM photographs of experimentalresults obtained for thin wafer dicing. The SEM photographs show aportion of a 10×4 mm² die cut out of a 100-μm thick silicon wafer. Thissize was chosen because it is a common size for microprocessor chips.The SEM photographs shown in FIGS. 12A and 12B correspond to 700 and 500passes, respectively, at a scan speed of about 7 m/s. The laser pulseenergy was set to 20 μJ in order to achieve the maximum laserpenetration depth in the target wafer. The maximum laser repetition ratewas 500 kHz due to the 10 W average power limit of the laser used inthese experiments. The laser spot size was 30 μm at 1/e². With theseparameters the laser does not completely penetrate the substrate. Inthese example experiments, the laser trench serves as a scribe which isfollowed by a mechanical break (along the scribe line) to complete diesingulation.

The SEM photographs in FIGS. 12A and 12B demonstrate that the edges arenearly free of cracks and show very little molten slag. Most of theparticulate debris left on the surface can be easily removed, forexample, in a short ultrasonic bath using a SharperTek SH80-D-2Lultrasonic cleaner applied for about 30 sec (see, e.g., thebefore-cleaning and after-cleaning results shown in FIG. 12B.)

Example Experiments with Longer Pulse Durations

Experimental data was obtained using longer pulse widths. FIGS.13A-1-13A-3 are SEM images showing results obtained with pulses having apulse width of about 200 ps. Repetition rates of 200 kHz, 350 kHz, and500 kHz were used at various scan speeds. In these experiments, thepulse compressor was detuned so as to produce pulse widths of about 200ps and pulse energy of 20 μJ. The experimental results were alsosurprising. The trend of reducing a quantity of unwanted materialcontinued in these experiments with longer pulse durations. However, incomparison to the experiments with ultrashort pulses, better featurequality, trench shape, and repeatability were obtained with ultrashortpulses.

FIGS. 13A-4-13A-5 are plots corresponding to the SEM images of FIGS.13A-1-13A-3. The measurement method for determining area was modified asdisclosed above (e.g., polygons were used). The plots in FIGS.13A-4-13A-5 suggest that predictability and repeatability of machiningresults may be affected at the longer pulse widths. FIG. 13A-5 isparticularly interesting. The ratio of ablated area to recast area isimproved at higher rep rates and can be discriminated from the 200 kHzdata. In various embodiments, the ratio of ablated area to recast areamay be greater than about 0.5, greater than about 1.0, greater thanabout 2.0, or some other value.

The experimental results of FIGS. 13A-1-13A-3 are summarized in thetables below.

Tables Corresponding to Results Shown in FIGS. 13A-1-13A-5 200 kHz 350kHz ablation recast ablation recast speed depth height speed depthheight (m/s) (um) (um) ratio (m/s) (um) (um) ratio 0.5 30 16 1.875 0.560 17 3.529412 1   17 10 1.7 1 30 14 2.142857 2   9 8 1.125 2 16 91.777778 5   4.5 4 1.125 5 16 5 3.2 7.5 4 3 1.333333 7.5 12 3 4 10   4 31.333333 10 9 3 3 200 kHz 350 kHz ablated recast ablated recast speedarea area speed area area (m/s) (um{circumflex over ( )}2)(um{circumflex over ( )}2) ratio (m/s) (um{circumflex over ( )}2)(um{circumflex over ( )}2) ratio 0.5 325 560 0.580357 0.5 990 5521.793478 1   297 325 0.913846 1 465 420 1.107143 2   126 220 0.572727 2264 247 1.068826 5   58.5 104 0.5625 5 208 112 1.857143 7.5 40 970.412371 7.5 156 67.5 2.311111 10   52 90 0.577778 10 112.5 75 1.5 500kHz ablation recast speed depth height (m/s) (um) (um) ratio 0.5 60 183.333333 1   45 14 3.214286 2   26 9 2.888889 5   13 4.5 2.888889 7.5 104 2.5 10   9 3.5 2.571429 500 kHz ablated recast speed area area (m/s)(um{circumflex over ( )}2) (um{circumflex over ( )}2) ratio 0.5 990 4502.2 1   742 350 2.12 2   403 225 1.791111 5   201 101 1.990099 7.5 15590 1.722222 10   139.5 70 1.992857

Observations Based on the Experimental Results

The experimental results disclosed herein are a function of manyco-dependent variables, e.g., scan speed, laser energy, laser power,pulse power density, spot diameter, spot overlap, pulse width,repetition rate, instantaneous burst repetition rate, fluence, number ofmachining passes, and so forth.

The experimental results demonstrate a surprising influence ofincreasing laser repetition rates from at least several hundred kHz toabout 1 MHz on reducing the amount of redeposited material. Furtherreduction of redeposited material may occur at repetition rates up toabout 5 MHz, and possibly up to about 10 MHz. However, the improvedcombination of processing efficiency and quality may degrade at veryhigh repetition rates (e.g., greater than about 10 MHz or about 20 MHz),and the corresponding average laser power would be very high. Therefore,in various laser processing applications, both the upper and lowerbounds of the range of repetition rate may be critical to avoid degradedperformance. Further, at the upper bound, below about 10 MHz, theprocessing may also avoid undesirable heat accumulation effects, inaddition to reduced accumulation of redeposited material

Similar results may be obtainable for at least other semiconductormaterials including, for example, GaAs. Similarly, benefits may beobtained for workpieces other than semiconductor substrates. Laserparameters may be further adjusted, for example wavelength, although itis generally known that the ablation threshold at certain ultrashortpulse widths, for example in a range from about 50 fs to about 1 ps, isless wavelength dependent than at longer pulse widths, for example 10 psto 1 ns pulse widths.

In accordance with various embodiments, laser-based processing may becarried out with a fluence above an ablation threshold of the materialto about 20-times the ablation threshold. For example, in someembodiments, a preferred range for fluence may be about 5 to about 15times the ablation threshold.

Embodiments of silicon machining may be carried out with about 1-30 μJof pulse energy, and typically 5-25 μJ for efficient and high qualityprocessing. Repetition rates are advantageously above several hundredkHz, for example greater than 500 kHz. A beneficial range may be about500 kHz to about 5 MHz, and may be in a range of 1 MHz to about 10 MHz.

Scan rates are somewhat dependent on spot size in certain embodiments.Spot sizes may be in the range from about 10 μm to about 100 μm and scanspeeds may be in a range from about 0.2 m/s to 20 m/s.

As described above, a high ratio of ablated volume to redeposited volumemay be obtained with various embodiments. The quality of the processingmay be obtained without substantially sacrificing processing efficiency.

Example Experimental Results—Die Strength

In addition to reduced debris, a significant improvement in die strengthrelative to UV nanosecond lasers may result with the use of ultrashortpulses in some implementations of the disclosed systems and methods.Experimental results obtained with bare 50 μm thick wafers suggestedsuch an improvement with appropriate pulse parameters.

FIGS. 14, 14A-1, and 14A-2 schematically illustrate some arrangementsfor die strength measurement. FIG. 14, is adapted in part from Li et al,“Laser dicing and subsequent die strength enhancement technologies forultra-thin wafer”, Electronic Components and Technology Conference,IEEE, (2007), pp 761-766. The stress (MPa) may be estimated as follows:

σ(stress)=3FL/2bh ²

where F (Newtons) is the breaking load, L (mm) is the span length, b issample width (mm) and h is the sample thickness (mm).

FIG. 14A-1 corresponds to a side view of a sample arranged in tensionwhere the sample is supported with the laser cut direction (depictedwith arrow) facing one point of the 3 point mount. The oppositearrangement of FIG. 14A-2 corresponds to a sample arranged incompression. The latter apparently corresponds with “active layerupwards” measurement configuration as shown in FIG. 3 of Li et. al.,FIG. 3, and discussed in the corresponding text.

Experimental Results—Die Strength of 50 μm Bare Wafers

The table below shows laser pulse parameters varied during ten wafercutting experiments. The “double pulse” experiments correspond to twopulses having a 20 ns spacing, with the pulse pairs being repeated atthe 500 KHz repetition rate. The 700 fs pulse width and 500 KHzrepetition rate were constant.

Flexure Maximum Flexure extension Flexure stress at at Maximum AveragePulse Pulse Rep Scan No Load Std Maximum Load Std Flexure Load Std powerduration energy rate Speed Pass Single/ U N Dev Mpa Dev (nm) Dev W fs μJkHz m/s number Double note 1 0.83 0.29 1491.7 516.3 1.81 0.69 5 700 10500 7.5 200 Single Partial cut 2 0.43 0.04 781.3 78.4 0.79 0.10 5 700 10500 5 750 Single Complete cut 3 0.28 0.06 495.8 114.7 0.74 0.21 5 700 10500 0.5 100 Single Complete cut 4 0.14 0.03 252.7 60.3 0.2 0.16 5 700 10500 0.1 14 Single Complete cut 5 0.56 0.20 1016.0 367.1 1.23 0.51 2.5700 5 500 5 3000 Single Partial cut 6 0.38 0.07 690.7 118.1 1.35 0.18 5700 5 500 5 3000 Double Complete cut 7 0.28 0.04 507.7 78.2 1.036 0.18 5700 10 500 5 700 Single Complete cut 8 0.34 0.06 617.1 104.8 1.284 0.275 700 10 500 5 1000 Single Complete cut 9 0.22 0.04 390.2 71.5 0.7670.22 10 700 10 500 5 500 Double Complete cut

FIG. 14B is a plot illustrating the results of the die strengthmeasurements (summarized in the above table), all performed incompression as shown in FIG. 14A-2. In the above table, the first columnis experiment number, the second column is maximum flexure load (in N),the third column is standard deviation of the maximum flexure load, thefourth column is flexure stress at maximum load (in MPa), the fifthcolumn is standard deviation of the flexure stress at maximum load, thesixth column is flexure extension at maximum flexure load (in nm), theseventh column is standard deviation of the flexure extension at maximumflexure load, the eighth column is average laser power (in W), the ninthcolumn is pulse duration (in fs), the tenth column is pulse energy (inμJ), the eleventh column is pulse rate (in kHz), the twelfth column isscan speed (in m/s), the thirteen column is number of passes, thefourteenth column indicates whether single or double pulses were used,and the fifteenth column provides notes on whether complete or partialcuts were made.

Experiments 1 and 5 demonstrate that the statistical distribution isrelatively large for the case of partial (incomplete) laser cuts.

Experiments 2-4 show that for complete cuts, the maximum flexure stresssignificantly decreases, from 781 to 252 MPa, with reduction in scanspeed, from 5 to 0.1 m/s.

Experiments 5 and 6 show that 5 μJ is insufficient pulse energy for areasonable processing rate, in some implementations, with a spot size ofabout 40 μm (1/e²). A complete cut required at least 3000 passes, witheither single or double pulses. However, experiments in the followingsection will show 5 μJ is sufficient to cut 50 μm silicon samples with areduced spot size of about 20 μm, corresponding to a 4-fold increase influence.

Experiments 7-9 demonstrate that the use of a double-pulse burst at 500kHz (using two 10 μJ pulses separated by 20 ns) results in weaker diestrength than for a single 10 μJ pulse at 500 kHz.

Observations Based on Experimental Results

The experimental results with 50 μm samples suggest at least someimprovements in both die break strength and significant reduction indebris generation relative to conventional nanosecond laser dicing byappropriately using a high repetition rate ultrashort pulse laser systemto dice 50-μm thick silicon wafers. The experiments suggest a 2-3 foldimprovement, or perhaps larger, in die strength may be obtainablerelative to reported UV DPSS results.

The experimental results demonstrate that the best die strength resultswere achieved (in these experiments) using a beam scanning system whichrasters the beam at high speed (>1 m/s) a sufficient number of times toachieve a complete cut.

The experimental results indicate the cut quality and die strength bothdegrade with significantly fewer passes and slower scan speeds for thisexperimental setup. Incomplete cuts are generally undesirable.

Also, to achieve practical processing speeds at the laser spot sizerelatively high pulse energy is needed. For example, with a 40 μm spotsize (1/e²) pulse energy of at least about 5 μJ was applied to the Sisamples. Wafer processing may generally be carried out with spot sizesin the range of about 15-40 μm, and may preferably be in the range ofabout 30-40 μm. A minimum fluence may be about 1 J/cm². Energy of 5 μJover a 40 μm spot size corresponds to about 0.4 J/cm², and correspondsto a minimum fluence in the above table. Other spot sizes, energies, andfluences may be used in other embodiments.

The highest die break strength is typically achieved using mechanicaldicing blades. However, the processing speed reduces significantly inproportion to the wafer thickness.

It is instructive to compare the results with published data regardingdie strength. Example comparisons of die strength measurements obtainedwith a DPSS UV laser and mechanical saw are available in: (a) inToftness et al., “Laser technology for wafer dicing and microviadrilling for next generation wafers”, Proc. SPIE Vol. 5713, pp 54-66(2005), and (b) Li et al, “Laser dicing and subsequent die strengthenhancement technologies for ultra-thin wafer”, Electronic Componentsand Technology Conference, IEEE, (2007), pp 761-766.

In Toftness et al, Section 3, “Thin Wafer Dicing” various aspects of thetwo approaches are discussed. Wafers with 75, 80, or 180 μm thicknesseswere tested according to SEMI standard G86-0303. Specifically, for 75micron and 3 point die strength comparison, 444 MPa and 280 MPa datawere obtained for saw and laser data respectively. The range of valuesfor the saw was quite wide compared to the laser distribution.

Li et al, pp 761-763, provides comparisons between blade and laserdicing die strength of 50 μm samples. Results were reported for 3 pointmeasurements obtained in both compression and tension. The resultssuggest little difference for blade results in compression or tension.However, as shown in FIG. 3 of Li et al., pulsed laser processing with355 nm UV produced very different results. In compression (“active layerupwards”), roughly 450 MPa was measured, roughly twice the strengthobtained in tension. The blade results were in the range of 600-700 MPa,exceeding the laser results in all cases. Therefore, the resultsindicate that processing with UV DPSS systems yielded die strengthvalues roughly 50% of typical results obtained with mechanical cutting.

Referring to FIG. 14C, which shows failure stress for compression andtension experiments, it appears the compression arrangement has beenfound to be less favorable for ultrashort processing (at least in termsof failure stress). This result further supports the fact thatultrashort pulse laser dicing leads to a different failure mechanismthan UV nanosecond laser due to significantly different nature of theprocessing.

Therefore, at least some results suggest that a worst case ultrashortmeasurement configuration (e.g., compression) provides an improvementwith respect to a best case configuration for DPSS systems.

Ultrashort processing at sufficiently fast rates may produce diestrength results comparable to, or perhaps slightly less, thanobtainable with mechanical cutting. In some cases, processing is carriedout with at least 500 kHz rep rates, spot sizes in the range of about20-40 μm, and pulse energies at least about 5 μJ.

Such ultrashort processing may produce die strength in a range of about400 MPa to at least 700 MPa, and in some cases larger values, forexample up to about 900 MPa or greater than 1000 MPa.

The above results indicate die strength may be improved with ultrashortprocessing. However, it is known that die strength may be affected byseveral factors. Also, some information suggests die strength is of lessoverall importance for some embodiments of a laser-based process thandebris reduction, particularly for processing patterned wafers.

Moreover, it is expected that die strength could be improved withultrashort processing at lower pulse energies than 5-20 μJ with a 40 μmspot size. However, because high throughput may be advantageous in someimplementations, such an approach is generally regarded as deficient forsome such implementations. One possible wafer processing systembeneficially may simultaneously provide for adequate die strength, lowdebris, and high throughput.

Moreover, as will be shown in the following experiments, low pulseenergy, in some cases, may produce other detrimental effects whencutting patterned wafers.

Example Experimental Results—Patterned Wafer Scribing/Cutting

Referring back to FIG. 1G-2, a top view of a patterned wafer isschematically illustrated with multiple materials and patternsoverlapping a laser processing path 127-b within street 127. Thematerial and patterns may be disposed to provide for electrical testingor other functions. Numerous combinations of materials may be presenthaving different thermal, optical, electrical, or mechanical properties.

For example, as generally illustrated in the example of FIG. 1G-2, amicroprocessor architecture may be complex and comprise various patternsand materials. Layers of metal, low-k dielectric patterns, functionalcircuitry formed on a fine grid, may all be supported on a siliconsubstrate and covered with an overlying passivation layer (not shown).

As earlier noted, materials may include, but are not limited to, metals,inorganic dielectrics, organic dielectrics, semiconductor materials,low-k dielectric materials, or combinations thereof. The combinations ofmaterials may be arranged in different spatial patterns and stacked indepth. For example, microelectronic circuits may comprise portionshaving alternating layers of copper and low-k material, covered by theoverlying passivation layer. Many possibilities exist for semiconductorarchitectures.

The experimental results below will demonstrate scribing through activematerial layers disposed within the streets without generatingsignificant material debris. Although it is difficult to directlymeasure the extent of a heat affected zone (HAZ), a general objective isto cleanly remove multiple materials, with negligible melting, and withno significant change to layer morphology.

Parameters used for cutting of bare silicon wafers in the above examplesprovide at least a useful starting point for patterned waferscribing/cutting. The example parameters used in the followingexperiments may provide good scribing performance of some patternedwafers. Other parameters may be used.

Example Experimental Results with Patterned Wafers/Multi-MaterialDevices

The following types of patterned wafers were studied:

Experiment 1: GaN on copper (LED device)

Experiment 2: a patterned microelectronic circuit,

Experiment 3: a microprocessor device, and

Experiment 4: a flash memory device.

Experiment 1

A particularly encouraging result was obtained with processing of GaN onCopper. The result was obtained with parameters that may be well suitedfor cutting several bare wafers: 10 μJ, 500 kHz, approximately 7 m/sec,with about 1000-1500 passes, and a spot size of about 30-40 μm (1/e²diameter). FIG. 15A is an SEM image showing a high quality cut 1505 withlittle or no debris. FIG. 15B is a side view of the cut. Variousmaterials are discernible in the image, including overlying material1510, GaN material 1515, and inner layer(s) 1520. No attempts were madeto clean the sample after laser processing.

Experiment 2

A microelectronic circuit having an overlying passivation layer,multiple alternating layers of copper and low-k dielectric, and asilicon substrate was laser scribed. Processing was first carried outwith 100 passes, about 7 msec scan speed, 10 μJ pulse energy, and 500kHz. FIGS. 15C and 15D are SEM images schematically illustratingincomplete cuts of a copper pad and some low-k delamination,respectively. The laser parameters resulted in removal of thepassivation layer, but only partial removal of the copper layer. Theregion of the cut also shows noticeable surface texturing. The low-kdielectric removal was incomplete with evidence of delamination andcracking 1530. In some cases, a reduction in scan speed, and acorresponding increase in spatial overlap of the pulses, may improvecopper removal.

Experiment 3A

A microprocessor device was processed with 10 μJ pulse energy and 500kHz repetition rate. The number of passes were 200, 100, and 50 atrespective scan speeds of approximately 7.0, 5.0, and 2.0 msec. SeveralSEM images (not shown) displayed variations in cut quality with scanspeed and number of scans. The particular number of passes was chosen tocompletely cut through passivation, metal, and dielectric layers, downto the underlying silicon substrate. The number of passes wasapproximately inversely proportional to the scan speed.

As previously observed, the least debris and HAZ were generated for thehighest scan speed. This was particularly evident from the differencesin cut width between a top layer and a buried grid layer within or nearthe laser path 127-b of street 127, as schematically illustrated in FIG.1G-2. By way of example, a grid layer 129 is schematically illustratedin FIG. 1G-2 and FIG. 1G-3, and in this is experiment was in the laserpath as shown. A scribing experiment resulted in the area of an exposedburied grid layer at the edge of a scribe being considerably larger forthe case of 50 passes with scan speed about 2.0 m/s vs. 200 passes atapproximately 7.0 m/s.

Therefore, in contrast to the results of Experiment 2 above, thisexample demonstrated typical parameters for bare wafer processing mayalso be suitable for processing a patterned wafer in some cases.

Experiment 3B

Another experiment was carried out with the laser parameters ofExperiment 3A, but with 200 passes at about 7 msec. In this experimentdelamination between the dielectric and metal layers was observed incertain regions within the “streets”. Such delamination can be asignificant problem in some applications, because the induced cracks canpropagate through the device after die singulation and ultimately mayeven cause device failure. Steps to reduce delamination are discussed inexperiment 3C below.

Experiment 3C

Further experiments showed delamination/cracking between the low-kdielectric and metal layers is affected by variation in scan speed inthe experimental system. Laser processing was carried out with 5 μJpulse energy at 500 kHz repetition rate. Only single passes were usedfor testing. Reducing the scan speed from a maximum of 10.0 m/s to 250mm/s reduced the delamination/cracking.

The effect of pulse energy was also studied, particularly for singlescan passes at 250 mm/s. A minimum of 2 μJ was necessary to ablate metaland non-metal areas of this sample in these experiments. However, 2 μJenergy caused more delamination/cracking than observed for 5 μJ and 10μJ under the same scan conditions. Moreover, the 5 and 10 μJ results fora single pass at 250 mm/s were compared. No apparent or significantdifference in delamination/cracking was found. The higher pulse energywas beneficial in the example by providing for a complete cut throughthe thickest metal regions, thereby providing the highest throughput.

Increased magnification was used to evaluate scribes formed in areashaving thick metal pads. The areas were scribed using 10 μJ pulses at500 kHz repetition rate. It was confirmed that a single pass at 250 mm/swas sufficient to completely cut through the passivation, metal, anddielectric layers, and to the base silicon substrate of this sample. Inthis example, with 500 KHz repetition rate, 40 μm spot size, and 250mm/sec scan speed, spot overlap is about 99%. The results showednegligible HAZ and minimal debris redeposition.

Moreover, the passivation layer for this sample was polyimide, athermally sensitive polymer. Although observations indicated thepassivation layer receded from the scribe region, no evidence ofcharring was found. Such charring is a common detrimental resultassociated with thermal laser processing effects.

A surprising result of these experiments was the dependence ofdelamination on both pulse energy and scan speed. A 2.5-fold increase inpulse energy improved the result, and a reasonable operating range wasfound at higher pulse energies up to at least 10 μJ. The minimum fluencewith approximately 5 μJ over a 40 μm spot was about 0.4 J/cm², with sucha spot size providing for high throughput.

Experiment 3D

In the previous experimental results described in Experiments 1-3C, thefocal spot diameter was 35-40 μm (1/e² diameter). In order to facilitateexperiments at 1 MHz, the spot diameter was reduced to 20 μm. As such, 5μJ over the 20 μm spot (e.g.: a fluence of 1.6 J/cm²) was sufficientlyhigh to completely scribe through active layers in the streets of themicroprocessor sample.

The increase in repetition rate to 1 MHz also allows a linear increasein scribing speed for this example. Delamination between the dielectricand metal layers was avoided with an optimal speed between about 400-500mm/s in this example. This was also a sufficiently fast scan speed toavoid possible problems with heat accumulation. By way of example, withtypical energy of about 5 μJ per pulse and a 500 KHz to 1 MHz pulserate, speed of about 0.2 msec to 1 msec may be suitable for cleanremoval of low-k dielectrics. At 1 MHz, and with speed of 0.5 msec andspot size of 20 μm, the approximate pulse overlap was about 98%.

In this experiment, the reduction of the spot diameter and the incidentpulse energy also reduced the cut width in both the active andpassivation layers. Furthermore, the difference in cut width between theactive and passivation layers allowed for steeper side walls within thecut.

Experiment 4

A flash memory device was processed. Such devices are also formed withmultiple materials in the streets, in some cases a fine grid. Thestructure included a thin silicon substrate (typically 50-75 μm thick)with metal and dielectric layers coated by a passivation layer.

Conventional mechanical dicing results exhibited obvious edge chipping,and some delamination of the dielectric layers.

Ultrashort pulses were used to cut through the full 50-μm waferthickness. Similar to Experiments 1 and 2, but unlike Experiment 3, onepossible preferred dicing method is to use many passes (e.g., 550 inthis case) at a relatively high translation speed (e.g., approximately7.0 m/s). A fewer number of passes (e.g., less than 550) could be usedto cut the entire wafer. The spot diameter was 20 μm as in Experiment3D. However the pulse energy was 10 μJ and the repetition rate was 500kHz as in Experiments 1-3C.

This experiment indicates that efficient cutting of full wafer thicknesswas demonstrated while limiting, if not minimizing, debris redepositionand HAZ. Delamination problems were not found in the experiment, whichmay at least in part be a result of the specific device construction. Itshould be noted that no post-processing was used to clean the sampleafter laser cutting.

Mechanical cutting typically uses a large amount of water to clean/coolthe blade during cutting. It is likely that the majority of the laserdicing debris can be removed by standard wafer spin rinse/dry systemswithout the need for special protective coating.

This flash memory application requires a complete cut through the wafer,and one concern is laser induced damage to the dicing tape. Inconventional nanosecond laser cutting, the optical/thermal penetrationof the laser into the tape is generally quite deep which cansignificantly reduce the tape strength and complicate subsequent “pickand place” of the die after singulation. In the case of nanosecond UVlaser dicing, there has been a large effort in the industry to developspecialized laser dicing tape which limits the depth of laserpenetration into the tape. With ultrashort pulse laser machining, it ispossible to choose parameters such as, e.g., the number of laser passesso as to completely cut through the substrate but not substantiallydamage the tape. The precise nature of ultrashort laser ablation reducesor eliminates the need for specialized tape, so that standard mechanicaldicing tape can still be used.

Observations Based on the Example Experimental Results

It may be desirable for laser parameters and scan speeds to be modifiedin order to achieve best scribing results.

Relatively high repetition rates, for example in the range of 500 kHz toabout 1 MHz, resulted in low debris, as observed with bare waferexperiments. Generally, sufficiently high repetition rates will avoidaccumulation of debris. However, an increase in repetition rates tovalues above several MHz, (for examples tens of MHz or higher) mayincrease thermal effects and HAZ induced material modification in somecases. It may be advantageous for some scribing/dicing implementationsfor the scan speed and laser spot size to be sufficiently large toprovide acceptable throughput.

Because of the variation in pattern construction and materials instreets between adjacent die, some complex wafer designs may requirerelatively more experimentation to identify process operatingparameters. Therefore, it may be advantageous for a laser and machiningsystem to provide for sufficient adjustment of laser parameters, forexample, pulse energy, scan speed, etc.

Some examples showed it may not be sufficient to set processingconditions for some implementations based upon only minimization ofdebris and HAZ in isolated area. In some cases parameters may beadjusted between passes to identify suitable process parameters. A verycomplex pattern design for a workpiece may limit the processing windowto a relatively narrow set of parameters within an adjustable range, ormay lead to some compromise in processing throughput.

Flexibility and adjustability of laser parameters over a wide rangeadvantageously may provide for processing of patterned wafers havingmultiple materials in the streets. Operating with microjoule pulseenergy, 500 kHz-1 MHz, over typical 20-40 μm spot sizes, and about0.2-10 m/sec was shown to be generally beneficial in these experiments.

A surprising dependence on a combination of pulse energy/fluence andspeed was found for processing certain patterned and bare waferportions. With fixed parameters, slower speeds tend to produce increasedpulse overlap and exposure of a wafer region, for example “topside”patterned wafer portions having multiple layers. Increased speed, anddecreased spatial overlap between spots, tends to be suitable for barewafer processing. Once an initial scribe is complete, the underlyingsubstrate (typically silicon) can be cut using a mechanical saw for someimplementations for thick wafers.

Alternatively, laser cutting parameters used for thin wafers may besuitable. In particular, for 100 μm, 75 μm, 50 μm or other thinsubstrates the same high repetition rate ultrashort pulse fiber laser of(e.g.: shown in FIG. 1F or FIG. 6A) may be used for the active-layerscribing (with typically a low number of relatively slow scans in somecases) and for the substrate cutting (with typically a large number ofrelatively high speed scans in some cases). Certain parametersincluding, e.g., pulse energy, repetition rate (e.g.: rate at whichpulses are applied to the substrate), and scan speed are advantageouslyadjustable over a wide range in some implementations.

The high degree of depth precision possible with some ultrashort laserpulse wafer dicing embodiments can be utilized and calibrated tocompletely cut the wafer material without significant cuttinginto/through the underlying dicing tape. As such, standard mechanicaldicing tape may be acceptable, whereas it is well known thatconventional nanosecond UV laser dicing requires the usage of speciallydesigned dicing tape.

Parameters for processing a patterned wafer may overlap or be distinctfrom typical bare wafer processing parameters. Therefore, a laserprocessing system that provides for adjustment of parameters over asufficiently wide range may be suitable for processing a wide variety ofsemiconductor substrates, both patterned and un-patterned.

Some experimentation generally is expected for different productiondesigns to optimize processing.

Some values and/or ranges for parameters for processing thin (e.g.: 50μm, 75 μm, etc.) patterned or non-patterned silicon wafers at a near IRwavelength may include some or all of the following in some advantageousembodiments:

Wavelength: approximately 1 μm

Number of passes: 10-1000 typical, up to about 1500

Spot size (1/e²): 10-50 μm, 20-40 μm typical

Pulse width: sub-picosecond (e.g.: >100 fs) to about 10 ps, less thanabout 50 ps

Pulse energy: about 2-20 μJ, 5-10 μJ typical, higher energies typical tolimit delamination, and to process copper with high throughput

Minimum fluence: greater than about 0.4 J/cm² (e.g.: about 5 μJ over 40μm spot 1/e² diameter)

Repetition rate: 500 kHz-5 MHz (delivered to target surface)

Scan speed: 0.1 msec to 10 msec, >1 msec typical for non-patternedwafer, <5 msec typical for patterned wafers, 0.2 m/s to 1 msec fortypical low-k materials

The above values and ranges are examples; other values and ranges arepossible in other embodiments.

In some embodiments one or more initial passes may be carried out at arelatively slow scan speed to remove metal and/or dielectric material,for example multiple layers. Additional passes may be carried out atincreased speed for cutting the semiconductor wafer, for example theunderlying silicon substrate that supports the metal and/or dielectriclayers.

By way of example, the first passes (e.g.: for dielectric/conductorremoval) may be carried out at about 0.2 msec to 1 msec using pulseenergy in the range of greater than about 2 μJ and up to about 10 μJ.The additional passes may be carried out at speed of up to about 10 msecwith pulse energy as above. A focused spot size may be in the range ofabout 20-40 μm (1/e² diameter). A minimum fluence may be about 0.4J/cm². Pulse widths may be about 10 ps or less. Other parameters for thefirst and/or additional passes are possible.

Example Experimental Results—Femtosecond and Picosecond Pulses

Scribing Example with fs and ps Pulses

Additional experiments were carried out to compare scribing resultsobtained with femtosecond and picosecond pulses. The systemconfiguration used was similar to the system schematically illustratedin FIG. 6A. In these experiments, the laser system 104 comprised a D-10Klaser made by IMRA America Inc. (Ann Arbor, Mich.).

Pulsed laser beams were generated with the D-10K laser, which wasconfigured with a pulse compressor that produced sub-picosecond outputpulses in some experiments. The output wavelength was 1.04 μm, and apulse train with 10 μJ energy per pulse was generated at 1 MHzrepetition rate. The pulse energy of the femtosecond and picosecondpulses was approximately equal. Before compressing, the laser pulseduration was about 300 ps, and corresponds to a stretched and amplifiedoutput of a mode-locked oscillator. In this experiment, the 300 pspulses were obtained by removing the pulse compressor. The compressedpulse width was about 500 fs. In one set of example experiments (e.g.,the example results shown at the left of FIG. 17), compressed pulseshaving about 5 μJ pulse energy were used. In all of these exampleexperiments, the laser beam was focused using an F-theta lens, and the100 μm thick silicon wafer was placed at or near the focal plane of thelens. The pulsed laser beams were scanned across the silicon wafer so asto scribe the wafer with multiple passes. For each pass, a laser beamwas scanned one time across the wafer with a scan speed discussed below.

FIGS. 16A-16D illustrate example scanning electron microscopy (SEM)images of an unpatterned silicon wafer scribed using 500 fs laser pulses(FIGS. 16A and 16C) and with 300 ps laser pulses (FIGS. 16B and 16D).The results illustrated in FIGS. 16A and 16C were obtained with scanspeeds about 120 mm/s, and the results in FIGS. 16B and 16D with scanspeeds of about 320 mm/s. The SEM images of laser scribed grooves shownin FIGS. 16A and 16B are side views of a cleaved (after scribing)surface. The SEM images shown in FIGS. 16C and 16D are top views ofportions of the grooves shown in FIGS. 16A and 16B, respectively.

In these example experiments, recast was much lower with femtosecondpulses than with picosecond pulses. The silicon sample, scribed with 500fs laser pulses, shows no observable recast region in FIGS. 16A and 16C.Thus, it was confirmed that only a very shallow HAZ was generated byfemtosecond laser pulses in this example experiment. However, scribingwith 300 ps pulses resulted in a noticeable recast region about thescribing groove in the example experimental results shown in FIGS. 16Band 16D. The recast (illustrated by arrows in FIG. 16B labeled “psRecast”) indicates melting of silicon, and larger HAZ with 300 ps pulsesthan with 500 fs pulses, in these experiments. The recast produced with300 ps pulses in the example shown in FIG. 16B is believed to be aresult of thermal melting of the redeposited material. In certainexample of femtosecond processing illustrated in earlier embodiments ofwafer processing, melting of the redeposited material may not occur (ormay occur to a lesser extent than with longer pulses, e.g., >100 pspulses). Without subscribing to or requiring any particular theory orexplanation, the recast of particulate debris associated withsub-picosecond laser ablation may involve somewhat different phenomenonthan, for example, the thermal melting that may occur for longer pulsedurations, e.g., as observed in some experiments using pulse widthsgreater than 100 ps. The uncompressed test results of the examplesdescribed with reference to FIGS. 13A-1 to 13A-5 (using 200 ps pulses)illustrate the quality of silicon wafer cutting may be degraded withlonger pulse widths in some cases. Nevertheless, the melting andrecasting observed in 300 ps processing was gentle (e.g., compared tonanosecond processing). For example, FIG. 16B does not show cracking inthe region of material modification.

The 300 picosecond example experimental results illustrated in FIGS. 16Band 16D show a smooth scribed surface compared to the example experimentwith sub-picosecond pulses illustrated in FIGS. 16A and 16C. Althoughfemtosecond pulses created a relatively flat scribing groove withrelatively shallow HAZ (see FIG. 16A), the scribing surface was full oftexture (see FIG. 16C). Without being limited to any particular theoryor explanation, it is believed the texture results from laser inducedperiodic surface structures (LIPSS). Scribing with 300 ps pulses formeda much smoother surface in these experiments, apparently a result of themelting process described above. On the other hand, the pulse width isshort enough in the illustrated example to avoid creating surfacevariations on a scale comparable to wafer features (e.g.: conductors,dielectric layers). Thus, 300 ps pulses provided much smoother scribedsurface quality than did 500 fs pulses, in these example experiments.Accordingly, in some implementations, longer pulse widths (e.g., longerthan about 100 ps in certain embodiments) may be used to produce asmooth and substantially texture-free surface portion of a metalmaterial, a dielectric material, and/or a semiconductor material in aworkpiece.

The experimental results also showed superior die strength is achievablewith either femtosecond pulses, or with picosecond pulses having pulsewidths of a few hundred picoseconds. For example, FIG. 17 shows diestrength of silicon dies cut with 500 fs (compressed D-10K outputpulses), and 300 ps (uncompressed D-10K output pulses). In FIG. 17,individual experimental results are shown as open circles (die intension) and open squares (die in compression). The average values (anderror bars) corresponding to the experimental results are shown asfilled circles (die in tension) or filled squares (die in tension),which are horizontally offset from the experimental results. Forcomparison, results of mechanical and nanosecond laser dicing disclosedin Li et al, “Laser dicing and subsequent die strength enhancementtechnologies for ultra-thin wafer”, Electronic Components and TechnologyConference, IEEE, (2007), pp 761-766 are shown. The example resultsillustrated in FIG. 17 indicate that dicing with 300 ps laser pulsesproduced die strength similar to those of dies cut with 500 fs laserpulses, and these experimental die strengths obtained from 300 ps and500 fs pulses are stronger than the die strength of dies cut with ananosecond laser.

Observations Based on the Example Experimental Results

In various embodiments, low-k dielectric scribing may be moreefficiently carried out with HAZ sufficiently large to cause materialmodification over a depthwise region that intersects multiple layers.The extent (e.g., a depthwise extent) of the HAZ advantageously may belimited to reduce or avoid cracking, voids, or substantial unwantedre-deposited material. Also, in some implementations, the system isconfigured such that HAZ generated during removal of a dielectricmaterial (e.g., a low-k dielectric) and/or a metal material in theworkpiece is increased depthwise relative to HAZ generated duringremoval of a portion of a semiconductor material of the workpiece. Forexample, in some implementations, the depthwise extent of the HAZgenerated during removal of the dielectric material and/or the metalmaterial may extend through (and/or intersect) multiple layers ofmaterial.

FIG. 16B illustrates an example of material removal with a thermalinteraction resulting in redeposited material, but is non-catastrophicwith the absences of cracking and voids. FIG. 16A illustrates an exampleof an ultrashort (fs) laser ablation mechanism, wherein a depthwiseportion of material is removed with reduced or negligible re-deposition(compared to the example in FIG. 16B). In these experiments with baresilicon, the presence of some recast without cracking or otherundesirable modification is an indicator that the HAZ is sufficient forremoval of an overlying layer of low-k material. Also, the experimentalresults again confirm the reduction or avoidance of recast, slag, moltenregions, etc. with femtosecond processing of a bare wafer, anddemonstrate a benefit of femtosecond pulses for cutting the entirethickness of the wafer, or a substantial portion thereof.

Debris accumulation may be further reduced in some micromachiningoperations with use of electrostatic attraction of charged particlesejected from the target material. U.S. Pat. No. 6,770,544, entitled“Laser Cutting Method”, discloses such a technique. A dust collectingelectrode which is positively or negatively charged may be installed inthe vicinity of the laser irradiator of a wafer cutting system or othermicromachining device. With this arrangement, charged fragments producedby laser irradiation can be electrostatically attracted by the dustcollecting electrode, thus preventing the charged fragments fromdepositing in the vicinity of the laser irradiator. Such methods may beutilized with various embodiments to further enhance performance. Thefigure of merit may depend, at least in part, on the relativedistribution of charged and neutral particles in the ejecta.

Additional Embodiments, Features, and Example Applications

As described herein, unwanted material may accumulate within the targetregion, proximate to the region, or both during processing of a targetsubstrate. Embodiments which reduce the quantity of redeposited materialand/or alter the composition of the debris may reduce or eliminateadditional processing steps. For example, for semiconductor processingthe quantity of unwanted material may be reduced sufficiently such thatconventional ultrasonic cleaning may be used to remove some or all ofthe unwanted material. Additionally, use of some embodiments of thelaser systems described herein may result in redeposition of fineparticles rather than “blobs” of material. In such embodiments, use ofchemical etching or other cleaning steps may not be required.

Numerous embodiments of the systems and methods described herein areapplicable for processing semiconductor substrates. Some embodimentsadvantageously may reduce or eliminate the need for special coatingand/or etching steps now utilized in the industry for debris removal.Some embodiments may provide additional and/or different advantages.Examples of certain additional embodiments are described herein. Theseadditional embodiments are intended to illustrate certain advantageousexamples of various systems and methods and are not intended to limitthe scope of the disclosure.

In one embodiment, a method of laser processing a workpiece is provided.The method may comprise focusing and directing laser pulses to a regionof the workpiece at a pulse repetition rate sufficiently high so thatmaterial is removed from the region and a quantity of unwanted materialwithin or proximate to the region is reduced relative to a quantityobtainable at a lower repetition rate. In at least some embodiments, theregion of the workpiece comprises a semiconductor wafer, and thequantity of unwanted material comprises redeposited material. In varioussuch embodiments, the redeposited material is limited to a thicknessless than about 20 μm, less than about 10 μm, less than about 5 μm, lessthan about 4 μm, or less than about 3.5 μm.

At least one embodiment includes a method of laser processing a targetmaterial to remove a depthwise portion of the material. The method maycomprise: repeatedly irradiating at least a portion of the targetmaterial with focused laser pulses at a scan rate and a pulse repetitionrate. The repetition rate is sufficiently high to efficiently remove asubstantial depthwise portion of material from a target location and tolimit accumulation of unwanted material within or proximate to thetarget location. In various embodiments, depth of the removed materialmay be greater than about 10 μm, greater than about 25 μm, greater thanabout 50 μm, greater than about 75 μm, greater than about 100 μm,greater than about 125 μm, greater than about 150 μm, or some otherdepth. In certain embodiments, depth of the removed material issufficient to cut entirely through a target material having a thicknessgreater than about 10 μm, greater than about 25 μm, greater than about50 μm, greater than about 75 μm, greater than about 100 μm, or someother depth. In some embodiments, the depthwise portion comprises arelatively shallow trench with a depth that may be, for example, lessthan about 10 μm, less than about 5 μm, or some other value. In variousembodiments, width of the removed material may be in a range from about5 μm to about 100 μm, in a range from about 10 μm to about 50 μm, in arange from about 20 μm to about 40 μm, or some other range.

At least one embodiment includes a method of processing a targetmaterial for at least one of cutting, dicing, scribing, or forming afeature on or within the target material. The method may compriserepeatedly irradiating the target material with focused laser pulses ata scan rate and a pulse repetition rate. The repetition rate may be in arange of at least about a 100 kHz to about 10 MHz in some cases. Thescan rate may be in the range of about 0.2 m/s to 20 m/s in some cases.The scan rate may be in the range of about 0.5 m/s to about 10 m/s insome cases. In certain embodiments, at least some of the focused pulseshave at least one of the following: a non-zero spatial overlap factorwith at least one other pulse, a pulse width below about 1 ns, a pulseenergy in a range of about 5 μJ to about 25 μJ, a focused 1/e² spot sizein a range of about 10 μm to about 50 μm. The pulses may produce afluence of about 0.25 J/cm² to about 30 J/cm² at the target material

In some implementations, the irradiating is carried out with multiplepasses over at least a portion of the target material. In someimplementations, at least a portion of the focused laser pulses removesat least a 5 μm depthwise portion of material from the target material.

In some implementations of a method of processing a multi-materialworkpiece, the workpiece comprises a semiconductor material and apattern, and the pattern comprises at least one of a dielectric materialand metal material. The method may include irradiating the workpiecewith a series of laser pulses. In some implementations, at least twopulses of the series have different characteristics that are applied todifferent materials of the workpiece. The method may also includecontrolling heat-affected zone (HAZ) such that at least one HAZgenerated during removal of at least one of the dielectric material andthe metal material is increased depthwise relative to at least one HAZgenerated during removal of a portion of the semiconductor material. Insome embodiments, at least some laser pulses have different pulsewidths, and controlling HAZ comprises applying different pulse widths tothe workpiece materials. The pulse widths can be in a range of about 100fs to about 500 ps. In some embodiments, the different characteristicscomprise at least one of: pulse energy, peak power, and spatial overlapat the workpiece. Controlling HAZ may comprise applying pulses having atleast one of the different characteristics to the different workpiecematerials. In at least one embodiment, at least one pulse of the seriesprovides fluence in a range from about 0.25 J/cm² to about 30 J/cm².

Embodiments of a system for at least one of dicing, cutting, scribing,and forming features on or within a material of a semiconductorsubstrate are described. The system may comprise a pulsed laser systemthat is configured to repeatedly irradiate at least a portion of thematerial with focused laser pulses at a scan rate and a pulse repetitionrate. The repetition rate can be sufficiently high to efficiently removea substantial depthwise portion of material from a target location andto limit accumulation of unwanted material proximate to the targetlocation. The repetition rate may be in a range from about 100 kHz toabout 5 MHz in some embodiments. The system may include an opticalsystem to deliver and focus the laser pulses and a beam positioningsystem configured to position the laser pulses relative to thesemiconductor substrate at the scan rate. The positioning system maycomprise at least one of an optical scanner and a substrate positioner.The system may also include a controller coupled to the laser system,the optical system, and the positioning system.

In some implementations, this system also includes a beam manipulatorcoupled to the laser system and the controller. The beam manipulator,the laser system, and the controller can be operable to obtain a signalindicative of a condition of at least one of the substrate and the lasersystem. The controller may be operable to produce a control signal toalter the condition of at least one of the laser system and thesubstrate.

In some implementations, the laser source comprises at least one of afiber laser, a fiber amplifier, a passive Q-switched microchip laser,and a mode locked oscillator. The system can be configured to produce atleast one pulse having a width in a range from about 50 fs to a fewnanoseconds at the repetition rate.

In some implementations, the system is configured to provide at leastone laser pulse having a pulse width less than about 10 ps. In otherimplementations, the system can be configured to provide at least onepulse with a pulse width in a range of about 50 fs to about 500 ps. Insome implementations, the pulsed laser system comprises at least one ofan ultrashort laser and an optical amplifier for amplifying ultrashortpulses. In some implementations, the system is configured to operatewith a pulse repetition rate less than about 10 MHz.

At least one embodiment includes a method of laser processing targetmaterial to form a high-aspect ratio feature in the target material, thefeature having a sufficiently large ratio of depth to width, and afeature depth of at least about 5 microns. The method may compriseirradiating the target material with a series of focused laser pulses ata scan rate and a pulse repetition rate. The method may further comprisecontrolling one or more characteristics of a series of laser pulses.Controlled pulse characteristics may include a fluence, a pulse energy,a non-zero spatial overlap factor with at least one other pulse of theseries, and a pulse width. The repetition rate and scan rate aresufficiently high such that the feature quality is improved relative tothe quality obtainable with substantially the same pulse characteristicsand scan rate at a lower repetition rate.

At least one embodiment includes a method of laser processing a targetmaterial. At least one embodiment comprises irradiating the targetmaterial with a series of focused laser pulses at a scan rate and apulse repetition rate. The method may further comprise controlling oneor more characteristics of a series of laser pulses. Controlled pulsecharacteristics of pulses of the series may include a fluence, a pulseenergy of at least about 5 microjoules, a non-zero spatial overlapfactor with at least one other pulse of the series, and a pulse width.Processing quality may be characterized with a measure of redepositedmaterial within or proximate to a quantity of material removed. Therepetition rate and/or the scan rate are sufficiently high such that themachining quality is improved relative to the quality obtainable withsubstantially the same pulse characteristics and scan rate at a lowerrepetition rate below about 1 MHz.

At least one embodiment includes a method of processing a targetmaterial to cut, dice, scribe, and/or form a feature on or within thetarget material. The target material may comprise a semiconductor,metal, or dielectric. For example, the target material may comprisesilicon. The dielectric may comprise a low-k dielectric. The method maycomprise irradiating the target material with a series of focused laserpulses at a scan rate and a pulse repetition rate. The method mayfurther comprise controlling one or more characteristics of a series oflaser pulses. Controlled pulse characteristics of pulses of the seriesmay include a fluence, a pulse energy of at least about 5 microjoules, anon-zero spatial overlap factor with at least one other pulse of theseries, and a pulse width. The energy of at least pulse one may be in arange from about 5 μJ to about 25 μJ, a pulse width may be less thanabout 1 ps, a focused spot size may be in a range from about 10 μm toabout 50 μm, producing a fluence in a range from about 0.25 J/cm² toabout 30 J/cm² at the substrate. In some embodiments, a repetition ratemay be in a range from at least about 500 kHz to about 10 MHz, and ascan speed may be in a range from about 0.2 msec to about 20 msec.

In various embodiments a pulse width is less than 1 ps.

In some embodiments a pulse width may be up to a few nanoseconds.

In some embodiments a sub-nanosecond pulse width may be less than 1 ns,for example 500 ps or less.

In some embodiments, pulse widths from tens of femtoseconds to about 500ps may be used.

In various embodiments a pulse width is sufficiently short to avoidundesirable microcracking or other defects within or near to a region oftarget material.

In various embodiments a pulse width is sufficiently short to limitaccumulation of unwanted material at the higher repetition rate.

In various embodiments a pulse width is sufficiently short such that afeature shape is formed within a pre-determined tolerance.

A pulse width sufficiently short to form a predetermined feature shapemay be less than about 100 ps.

In various embodiments an upper limit for a repetition rate may be about2.5 MHz, about 5 MHz, or about 10 MHz, and may be selected so as toavoid one or more of heat accumulation effects and accumulation ofredeposited material.

In various embodiments a higher repetition rate may be less than about2.5 MHz.

In various embodiments a higher repetition rate may be less than about 5MHz.

In some embodiments a higher repetition rate may be less than about 10MHz.

At least one embodiment includes an ultrashort pulse laser systemsuitable for carrying out any of the embodiments of the methods of laserprocessing described herein.

At least one embodiment includes an ultrashort pulse laser system thatcomprises at least one of a fiber amplifier or a fiber laser.

A depthwise portion of material removed may be about 0.5 μm or greaterduring a single pass.

A repetition rate may be at least about 1 MHz

A cross-section of unwanted material may be limited to a sub-microndimension.

Feature quality may be measurable as a depth Z of a formed featurerelative to a measure of re-deposited material proximate to the feature.

In at least one embodiment the repetition rate may be at least about 500kHz to about 5 MHz, and the lower repetition rate in the range of about10 kHz to about 250 kHz

In at least one embodiment the repetition rate may be about 500 kHz to10 MHz, and a lower repetition rate in the range of about 10 kHz to lessthan about 400 kHz.

In at least one embodiment, the repetition rate may be about 500 kHz toabout 2.5 MHz, and a lower repetition rate may be in a range of about 10kHz to less than about 400 kHz.

The repetition rate may be in the range of at least about 500 kHz toabout 10 MHz, and the average power of pulses during active processingmay be as low as about 2.5 W.

At least one embodiment includes an ultrashort pulse laser systemsuitable for carrying out embodiments of the above-described method offorming high aspect ratio features.

The target material may comprise a semiconductor such as, for example,silicon.

The pulse energy may be at least about 1 microjoule, and sufficientlyhigh such that a fluence exceeds an ablation threshold of the targetmaterial over at least a portion of a focused 1/e² spot diameter.

The irradiating may be carried out in multiple passes over the material,and a depthwise portion of the feature depth may be formed during anypass.

In at least one embodiment a repetition rate may be at least 1 MHz.

The fluence on target material, pulse duration, and laser spot overlapmay be held approximately constant.

A measure of quality may include at least one of the following: averageheight, volume, and area of redeposited material over a region proximateto target material.

A measure of quality may include at least one of the following: peakheight, average height of a cross section within the region.

A further measure of quality may also include a ratio of the depth of afeature formed in the target material to any of the above qualitymeasures.

In various embodiments an approximate reduction in the amount of recastmaterial may include an apparent reduction in the recast particle size.

The number of passes N may be in a range from about 10 passes to about1000 passes.

A fluence may be about 5 times to about 20 times above an ablationthreshold of the material.

A fluence may be in a range of about 0.25 J/cm² to about 30 J/cm².

A pulse width may be below about 1 ps.

A scan rate may be about 10 m/s and a focused pulse may comprise a 1/e²spot size of about 10 microns to about 100 microns

In some embodiments the target material may comprise a silicon wafer,and the machining may comprise wafer scribing or dicing.

In some embodiments a quantity of redeposited material may besufficiently low to eliminate a processing step utilized to removeredeposited material of a larger quantity.

In some embodiments a quantity of redeposited material may besufficiently low such that ultrasonic cleaning removes redepositedmaterial, and without a requirement for a substrate coating or chemicaletching.

In some embodiments a repetition rate may be up to about 10 MHz andaverage power at least about 25 W, and up to about 100 W.

Spatial overlap of spots may be in a range from about 10% to about 50%.

In some embodiments, the power density of a pulse may be in a range fromabout 10¹² to about 10¹⁴ W/cm², and the pulse may have a width less thanabout 10 ps.

In some embodiments, the power density of a pulse may be in a range fromabout 10¹⁰ to about 10¹³ W/cm², and the pulse may have a width less thanabout 500 ps.

At least one embodiment includes a laser based system for scribing,dicing, or similar processing of multi-material workpiece. The workpieceincludes a semiconductor portion, for example a semiconductor substrate.The system comprises: a source of optical pulses. An opticalamplification system, comprising at least one large-mode fiberamplifier, amplifies a pulse from the source to an energy of at leastabout 1 μJ, and generates output pulses having at least one pulsewidthin the range of about 500 fs to a few hundred ps. The system includes amodulation system, including at least one optical modulator, thatadjusts the repetition rate of pulses delivered to the surface withinthe range of about a few hundred KHz to about 10 MHz. A beam deliverysystem delivers focused spots over a spot size (1/e²) of about 5-50 μmon one or more materials, and a scanning system is used to scan thefocused spots at a rate of about 0.1 msec to 20 msec. In someembodiments, the spot size (1/e²) may be in a range from about 15-50 μm.

Various embodiments may also comprise: a fiber-based chirped pulseamplification system having a pulse stretcher disposed between thesource and the large core amplifier, a pulse compressor that reduces apulse width of a pulse amplified with the large core fiber. Someembodiments include an optical amplification system operable to producean output pulse with pulse energy of about 20 μJ, with average power ofabout 10 W, a fiber oscillator, and one or more high gain amplifiersreceiving pulses from the oscillator, configured as an all-fiber design.

In various embodiments, the system may include:

a source of optical pulses having a mode-locked fiber oscillator;

a source having at least one of a fiber laser, a fiber amplifier, apassive Q-switched microchip laser, and a mode locked oscillator;

a pulse compressor that reduces the width of pulses emitted from theoptical amplifier;

a pulse stretcher disposed between the source and the optical amplifier;

the pulse stretcher may include a length of optical fiber;

a fluence may be at least about 0.25 J/cm² within a spot area, or atleast about 1 J/cm², and may be material dependent;

a spot size (1/e² diameter) may in the range of about 30-40 μm;

a pulse energy is in the range of about 1 μJ to about 20 μJ.

Various embodiments of a pulsed laser system may comprise: a source ofoptical pulses, and an optical amplification system, comprising at leastone large-mode fiber amplifier, that amplifies a pulse from the sourceto an energy of at least about 1 μJ, and generates ultrashort outputpulses having at least one pulsewidth in the range of about 100 fs toabout 1 ps. The system is preferably adjustable to deliver output pulsesat a repetition rate within the range of at least about a few hundredkHz to about 10 MHz.

Various embodiments may also comprise:

available average power of at least about 10 W;

a fiber-based chirped pulse amplification system;

a large-mode fiber amplifier having at least one of a multimode fiberamplifier, a large-core leakage channel fiber (LCF), a photonic crystalfiber (PCF), and a photonic bandgap fiber (PBGF). One or more of theamplifiers may be configured in such a way that a nearly diffractionlimited beam is output.

At least one embodiment comprises a method of scribing, dicing, orsimilar processing of a multi-material workpiece having a semiconductormaterial portion. The method includes: irradiating at least one materialof the workpiece with laser pulses having a pulsewidth in the range ofabout 500 fs to a few hundred ps, and at a rate of a few hundred kHz toabout 10 MHz. The pulses are focused into spots sizes of about 15-50 μm(1/e²), and the focused spots scanned at a rate of about 0.1 msec to 20msec on or within the at least one material. The irradiating controlsheat accumulation within one or more materials in such a way thatprovides for rapid material removal, while simultaneously limitingaccumulation of debris about the processed area, with control of aheat-affected zone (HAZ).

In various embodiments:

the workpiece thickness is less than about 100 μm;

the workpiece is formed with both a patterned layer and a baresemiconductor wafer portion. The patterned layer may have at least oneof a dielectric and metal material.

For processing some materials, the scanning speed for removal of atleast a portion of the patterned layer may be substantially less than ascanning speed for removal of the bare wafer portion. In someembodiments, an overlap between adjacent focused spots may besubstantially greater for irradiation of the patterned layer than forirradiation of the bare wafer portion. Different spot sizes may be usedfor illumination of the patterned layer than for illumination of thebare wafer portion.

Removal of the patterned wafer portions may be carried out with spotoverlap of at least about 95%. The spot overlap may be greater thanabout 99% in some embodiments.

The pulse energy may be in the range of about 1 μJ to about 20 μJ.

The patterned portion may be scanned at a rate of about 0.1-0.5 msec.

A pulse energy may be at least about 1 μJ, and a fluence on or within aconductor or dielectric material may be sufficiently high to avoiddelamination of the dielectric material.

Processing of some substrates may be carried out with fluence forremoval of the patterned layer exceeding the fluence for removal of abare wafer portion. In some implementations, heat accumulation forremoval of at least some of the patterned portion exceeds heataccumulation for removal of at least some of the semiconductor wafer. Insome such implementations, pulse energy, pulse width, repetition rate,fluence, spot overlap, and/or scan rate may be varied to providecontrolled heat accumulation in one or more regions of the workpiece.

At least one embodiment includes a method of laser processing aworkpiece. The method includes focusing and directing laser pulses to aregion of the workpiece at a pulse repetition rate sufficiently high sothat heat accumulation within one or more materials is controlled insuch a way that provides for rapid material removal, whilesimultaneously limiting accumulation of redeposited material about theprocessed area, with control of a heat-affected zone (HAZ).

Various embodiments may comprise a laser based system for scribing,dicing, or similar processing of multi-material workpiece having asemiconductor material portion. The system includes a source of opticalpulses, and an optical amplification system. The amplification systemcomprises at least one large-mode fiber amplifier that amplifies a pulsefrom the source, and generates output pulses having at least onepulsewidth in the range of about 500 fs to a few hundred ps. In otherembodiments, the amplification system may be configured to generateoutput pulses having at least one pulse width in a range from tens offemtoseconds to about 500 picoseconds. The system also comprises amodulation system, including at least one optical modulator, foradjusting the repetition rate of pulses delivered to the surface towithin the range of at least about 1 MHz to less than 100 MHz. A beamdelivery system delivers focused pulses over a spot diameter (1/e²) ofat least about 5 microns on one or more materials. A scanning system,comprising at least one beam deflector, scans the focused pulses at ascanning rate that produces a spot overlap of at least about 95% at therepetition rate and the spot size.

In various embodiments:

At least some of the output pulses have pulse energy of at least about100 nJ.

The spot overlap may exceed about 99%.

The source and amplification system may be all-fiber.

The amplification system may comprise a fiber-based chirped pulseamplifier.

In some embodiments of the laser-based system is configured such that:

A first output pulse has a pulsewidth greater than about 10 ps and asecond output pulse has a pulsewidth less than 1 ps.

The first output pulse and the second output pulse are overlapped intime.

The first output pulse and the second output pulse are separated in timeby less than about 1 μs.

The first output pulse is output when the scanning rate is at a firstrate, the second output pulse is output when the scanning rate is at asecond rate, the first rate less than the second rate.

In at least one embodiment, a multi-material workpiece may comprise botha patterned region and a semiconductor wafer region, the patternedregion having at least one of a dielectric and a metal material.Embodiments of methods of processing the workpiece may include some ofthe following: modifying at least a portion of material within thepatterned region with pulses having pulse widths in the range of about100 ps to about 500 ps, and modifying at least a portion of thesemiconductor wafer region with pulses having pulse widths in the rangeof about 100 fs to about 10 ps. In some embodiments, at least one pulsecomprises a pulse width in the range of about 100 ps to 500 ps, and atleast one pulse comprises a pulse width less than about 10 ps. In someembodiments, at least one pulse has a pulse energy of at least about 100nJ. In some implementations, the pattern comprises both a dielectricmaterial and a metal material, and heat accumulation within at least aportion of the pattern is sufficiently high to reduce or avoiddelamination of the dielectric material from the metal material. In someembodiments, a depthwise portion of a heat-affected zone (HAZ) producedby modifying at least a portion of the pattern is larger than adepthwise portion of a HAZ produced by modifying at least a portion ofthe semiconductor wafer.

Various embodiments of the methods for material processing describedherein may be implemented using at least some of the embodiments of thepulsed laser systems described herein. In various embodiments, thepulsed laser systems can comprise at least one of a fiber amplifier or afiber laser. For example, embodiments of the methods for materialprocessing may be implemented using embodiments of the systems shown anddescribed with reference to FIGS. 1F, 2A, 2B, 3, 4A, 4B, 5, 6A, and/or6B, and/or other pulsed laser systems. In some implementations, thelaser system (or components thereof, such as an oscillator and/oramplifier) may be implemented using an all-fiber design.

The example experiments, experimental data, tables, graphs, plots,photographs, figures, and processing and/or operating parameters (e.g.,values and/or ranges) described herein are intended to be illustrativeof operating conditions of the disclosed systems and methods and are notintended to limit the scope of the operating conditions for variousembodiments of the methods and systems disclosed herein. Additionally,the experiments, experimental data, calculated data, tables, graphs,plots, photographs, figures, and other data disclosed herein demonstratevarious regimes in which embodiments of the disclosed systems andmethods may operate effectively to produce one or more desired results.Such operating regimes and desired results are not limited solely tospecific values of operating parameters, conditions, or results shown,for example, in a table, graph, plot, figure, or photograph, but alsoinclude suitable ranges including or spanning these specific values.Accordingly, the values disclosed herein include the range of valuesbetween any of the values listed or shown in the tables, graphs, plots,figures, photographs, etc. Additionally, the values disclosed hereininclude the range of values above or below any of the values listed orshown in the tables, graphs, plots, figures, photographs, etc. as mightbe demonstrated by other values listed or shown in the tables, graphs,plots, figures, photographs, etc. Also, although the data disclosedherein may establish one or more effective operating ranges and/or oneor more desired results for certain embodiments, it is to be understoodthat not every embodiment need be operable in each such operating rangeor need produce each such desired result. Further, other embodiments ofthe disclosed systems and methods may operate in other operating regimesand/or produce other results than shown and described with reference tothe example experiments, experimental data, tables, graphs, plots,photographs, figures, and other data herein.

Other systems, setups, and parameters may be used in otherimplementations, which may provide the same or different results. Manyvariations are possible and are contemplated within the scope of thisdisclosure. Films, layers, components, features, structures, and/orelements may be added, removed, or rearranged. Additionally, process ormethod steps may be added, removed, or reordered.

Certain processing steps or acts of the methods disclosed herein may beimplemented in hardware, software, or firmware, which may be executed byone or more general and/or special purpose computers, processors, orcontrollers, including one or more floating point gate arrays (FPGAs),programmable logic devices (PLDs), application specific integratedcircuits (ASICs), and/or any other suitable processing device. Incertain embodiments, one or more functions provided by a controller or acontrol means may be implemented as software, instructions, logic,and/or modules executable by one or more processing devices. In someembodiments, the software, instructions, logic, and/or modules may bestored on computer-readable media including storage media implemented ona physical storage device and/or communication media that facilitatestransfer of information. In various embodiments, some or all of thesteps or acts of the disclosed methods may be performed automatically byone or more processing devices. Many variations are possible.

For purposes of this disclosure, certain aspects, advantages, and novelfeatures are described herein. It is to be understood that notnecessarily all such advantages may be achieved in accordance with anyparticular embodiment. Thus, for example, those skilled in the art willrecognize that the systems and methods may be embodied or carried out ina manner that achieves one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein. Furthermore, embodiments may include several novelfeatures, no single one of which is solely responsible for theembodiment's desirable attributes or which is essential to practicingthe systems and methods described herein. Additionally, in any method orprocess disclosed herein, the acts or operations of the method orprocess may be performed in any suitable sequence and are notnecessarily limited to any particular disclosed sequence.

Conditional language used herein, such as, among others, “can,” “could,”“might,” “may,” “e.g.,” and the like, unless specifically statedotherwise, or otherwise understood within the context as used, isgenerally intended to convey that certain embodiments include, whileother embodiments do not include, certain features, elements and/orsteps. Thus, such conditional language is not generally intended toimply that features, elements and/or steps are in any way required forone or more embodiments or that one or more embodiments necessarilyinclude logic for deciding, with or without author input or prompting,whether these features, elements and/or steps are included or are to beperformed in any particular embodiment. The terms “comprising,”“including,” “having,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list.

While certain embodiments of the inventions disclosed herein have beendescribed, these embodiments have been presented by way of example only,and are not intended to limit the scope of the inventions disclosedherein. Reference throughout this disclosure to “some embodiments,” “anembodiment,” or the like, means that a particular feature, structure,step, process, or characteristic described in connection with theembodiment is included in at least one embodiment. Thus, appearances ofthe phrases “in some embodiments,” “in an embodiment,” or the like,throughout this disclosure are not necessarily all referring to the sameembodiment and may refer to one or more of the same or differentembodiments. Indeed, the novel methods and systems described herein maybe embodied in a variety of other forms; furthermore, various omissions,substitutions, equivalents, and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions disclosed herein.

1. A method of scribing, dicing, cutting, or processing to remove material from a region of a multi-material workpiece, said method comprising: directing laser pulses toward at least one material of a multi-material workpiece, the laser pulses having a pulse width in a range from tens of femtoseconds to about 500 picoseconds and a pulse repetition rate of a few hundred kHz to about 10 MHz, the workpiece comprising both a pattern and a semiconductor wafer, said pattern comprising at least one of a dielectric material and a metal material; focusing said laser pulses into laser spots having spot sizes in a range from a few microns to about 50 μm (1/e²); and positioning said laser spots relative to said at least one material at a scan speed such that an overlap between adjacent focused spots for removal of material from at least a portion of the pattern is substantially greater than an overlap between adjacent focused spots for removal of material from at least a portion of the semiconductor wafer, wherein said method controls heat accumulation within one or more materials of said workpiece, while limiting accumulation of redeposited material about the region.
 2. The method of claim 1, wherein a thickness of said semiconductor wafer is less than about 100 μm.
 3. The method of claim 1, wherein at least one laser pulse has a pulse energy in a range from about 1 μJ to about 20 μJ.
 4. The method of claim 1, wherein said spot sizes are in a range from about 15 μm to about 50 μm, and said scan speed for removal of material from said pattern is in a range from about 0.1 msec to about 0.5 msec.
 5. The method of claim 1, wherein said laser pulses are output by an ultrashort pulsed laser system.
 6. The method of claim 1, wherein at least one laser pulse has a pulse energy of at least about 100 nJ, said pattern comprises said metal material and said dielectric material, and heat accumulation within said at least a portion of said pattern is sufficiently high to avoid delamination of said dielectric material from said metal material.
 7. (canceled)
 8. The method of claim 1, wherein a pulse width for removing at least a portion of said pattern is in a range from about 100 ps to about 500 ps, and a pulse width for removing at least a portion of said wafer is in a range from about 100 fs to about 10 ps.
 9. The method of claim 1, wherein the scan speed for removal of at least a portion of said pattern is substantially less than the scan speed for removal of at least a portion of said wafer.
 10. The method of claim 1, wherein said scan speed is in the range of about 0.1 msec to about 10 msec.
 11. (canceled)
 12. (canceled)
 13. (canceled)
 14. (canceled)
 15. (canceled)
 16. (canceled)
 17. A method of processing a multi-material workpiece, said workpiece comprising a semiconductor material and a pattern, said pattern comprising at least one of a dielectric material and metal material, said method comprising: irradiating said workpiece with a series of laser pulses, at least two pulses of the series having different characteristics that are applied to different materials of said workpiece; and controlling heat-affected zone (HAZ) such that at least one HAZ generated during removal of at least one of the dielectric material and the metal material is increased depthwise relative to at least one HAZ generated during removal of a portion of said semiconductor material.
 18. The method of claim 17, wherein at least some laser pulses have different pulse widths, and controlling HAZ comprises applying different pulse widths to said workpiece materials, said pulse widths in a range of about 100 fs to about 500 ps.
 19. The method of claim 17, wherein the different characteristics comprise at least one of: pulse energy, peak power, and spatial overlap at said workpiece, and said controlling HAZ comprises applying pulses having at least one of the different characteristics to said different workpiece materials.
 20. The method of claim 17, wherein at least one pulse of the series provides fluence in a range from about 0.25 J/cm² to about 30 J/cm².
 21. A method of processing a workpiece comprising both a pattern and a semiconductor wafer region, said pattern comprising a dielectric material and a metal material, said method comprising: modifying at least a portion of said pattern with focused laser pulses, at least one focused pulse comprising a pulse width in a range of about 100 fs to about 500 ps; and accumulating sufficient heat in said portion of said pattern to avoid delamination of said dielectric material from said metal material.
 22. (canceled)
 23. The method of claim 21, wherein at least one focused laser pulse has a pulse energy at least about 1 μJ.
 24. The method of claim 21, wherein said applied fluence is at least about 0.4 J/cm².
 25. The method of claim 21, wherein said pulse width is in a range from about 100 fs to about 10 ps and at least one focused laser pulse has a pulse energy in a range from about 1 μJ to about 10 μJ, and said method further comprises positioning said pulses relative to said pattern so that a spatial overlap between adjacent focused pulses exceeds about 95%.
 26. (canceled)
 27. (canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled)
 31. (canceled)
 32. (canceled)
 33. (canceled)
 34. (canceled)
 35. (canceled)
 36. (canceled)
 37. A laser-based system for scribing, dicing, cutting, or processing of a multi-material workpiece having a semiconductor material, the system comprising: a source of optical pulses; an optical amplification system configured to amplify a pulse from the source and to generate output pulses having at least one pulse width in a range from tens of femtoseconds to about 500 picoseconds; a modulation system, including at least one optical modulator, configured to provide a repetition rate of the output optical pulses to be in a range from at least about 1 MHz to less than about 100 MHz; a beam delivery system configured to focus and deliver pulsed laser beams to the workpiece, wherein a pulsed beam is focused into a spot size (1/e²) of at least about 5 microns; and a positioning system configured to scan said beams at a scan rate that produces a spot overlap on or within the one or more materials of the workpiece, the spot overlap at least about 95% at said repetition rate and said spot size.
 38. The laser-based system of claim 37, wherein at least some of said output pulses have a pulse energy of at least about 100 nJ.
 39. (canceled)
 40. (canceled)
 41. (canceled)
 42. The laser-based system of claim 37, wherein said amplification system comprises at least one large-mode fiber amplifier.
 43. (canceled)
 44. The laser-based system of claim 37, wherein said optical amplification system is configured to amplify a pulse from the source to an energy of at least about 1 μJ and to generate ultrashort output pulses having at least one pulse width in a range of about 100 fs to about 10 ps, said optical amplification system comprising at least one large-mode fiber amplifier, said at least one large-mode fiber amplifier comprising at least one of a doped large-core leakage channel fiber amplifier, a photonic crystal fiber, and a photonic bandgap fiber, wherein at least one fiber amplifier is configured such that said laser-based system emits nearly diffraction limited pulsed output beams; and wherein said laser-based system is configured to be adjustable to generate said pulsed output beams at a repetition rate in a range from about a few hundred kHz to about 10 MHz.
 45. A system for dicing, cutting, scribing, or forming features on or within a workpiece having a semiconductor material, said system comprising: a pulsed laser system configured to repeatedly irradiate at least a portion of said material with focused laser pulses at a scan rate and a pulse repetition rate, wherein said repetition rate is in a range of about 100 kHz to about 5 MHz and sufficiently high to efficiently remove a substantial depthwise portion of material from a target location and to limit accumulation of unwanted material about the target location; a beam delivery system configured to focus and deliver said laser pulses; a positioning system configured to position said laser pulses relative to said semiconductor substrate at said scan rate, said positioning system comprising at least one of an optical scanner and a substrate positioner; and a controller configured to be coupled to said pulsed laser system, said beam delivery system, and said positioning system, said controller configured to control a spatial overlap between adjacent focused laser pulses during processing of the workpiece at said repetition rate.
 46. The system of claim 45, further comprising a beam manipulator configured to be coupled to the laser system and the controller, said beam manipulator, said laser system, and said controller operable to obtain a signal indicative of a condition of at least one of said substrate and said laser system, and to produce a control signal to alter said condition of at least one of said substrate and said laser system.
 47. (canceled) 